Imperative Mood

For -ER verbs:

The imperative conjugations for nous and vous are the same as the present indicative, minus the final s for the tu form.

Verbs which are conjugated like -ER verbs (the indicative tu ends in -es), such as ouvrir and souffrir, follow the same rules as -ER verbs and drop the final s for tu. 

The imperative conjugations for all regular and most irregular -IR and -RE verbs are the same as the present indicative conjugations (meaning keep the s for tu).

Four irregular imperative verbs:
avoir
   (tu) aie
   (nous) ayons
   (vous) ayez

être
   (tu) sois
   (nous) soyons
   (vous) soyez

savoir
   (tu) sache
   (nous) sachons
   (vous) sachez

vouloir
   (tu) veuille
   (nous) n/a
   (vous) veuillez

http://french.about.com/od/grammar/a/imperative_2.htm

Future Tense

For future tense, all we need to do is use the infinitive form as root for -ER or -IR verbs and for -RE verbs, remove the final -e and add the future tense endings:

je          -ai
tu          -as
il           -a
nous     -ons
vous     -ez
ils         -ont

Infinitive	Future Root	Meaning	Similar Behaving Words
acheter	achèter-	to buy	achever, amener, emmener, lever, promener
acquérir	acquerr-	to acquire	conquérir, s'enquérir
appeler	appeller-	to call	épeler, rappeler, renouveler
aller	ir-	to go
avoir	aur-	to have
courir	courr-	to run	concourir, discourir, parcourir, more
devoir	devr-	must
envoyer	enverr-	to send
essayer	essaier-	to try	balayer, effrayer, payer
essuyer	essuier-	to wipe	appuyer, ennuyer
être	ser-	to be
faire	fer-	to do, to make
falloir	faudr-	have to
jeter	jetter-	to throw	feuilleter, hoqueter, projeter, rejeter
nettoyer	nettoier-	to clean	employer, noyer, tutoyer, more
pleuvoir	pleuvr-	to rain
pouvoir	pourr-	can
savoir	saur-	to know
tenir	tiendr-	to hold	maintenir, obtenir, soutenir
valoir	vaudr-	to be worth
venir	viendr-	to come	devenir, parvenir, revenir
voir	verr-	to see	revoir
vouloir	voudr-	to want

express need or obligation

the essential semi-auxiliary verb devoir, which means "must", "have to", or "need to" when placed before another verb.

• Je dois manger plus de légumes. — I must eat more vegetables.
• Ils doivent acheter plus de livres. — They need to buy more books.

Remember that the impersonal expression il faut + infinitive can also express a need or obligation.

• Il faut manger. — It is necessary to eat.
• Il nous faut payer des taxes. — We have to pay taxes.

You can also use the impersonal construction from the last unit, il est + adj + de.

• Il est nécessaire de reprendre le travail. — It is necessary to return to work.
• Il est nécessaire de faire attention. — It is necessary to pay attention.

Another way to express obligation is avoir à, though this is rarely used by French speakers because it tends to create vowel conflicts.

• Je sais ce que j'ai à faire. — I know what I have to do.
• J'ai quelque chose à vous dire urgemment. — I have something to tell you urgently.

What about when you want to say that you need something (instead of having to do something)? One way you learned previously is to use il faut with a noun instead of a verb.

• Il faut du lait. — Milk is needed.
• Il faut un début à tout. — A beginning is needed for everything.

A more common expression for need is avoir besoin de quelque chose. While this literally translates as "to have need of something", a better translation is "to need something".

• J'ai besoin d'un stylo. — I need a pen.
• Il a besoin d'eux. — He needs them.
• Elles ont besoin de magazines en anglais. — They need magazines in English.
• Vous avez besoin de ce produit. — You need this product.

You can also use this expression with verbs.

• Vous avez besoin de gagner plus d'argent. — You need to earn more money.
• Mes amis ont besoin de manger. — My friends need to eat.

Notice that besoin is invariable in this expression, but the noun besoin ("need") is just a standard masculine noun that does have a plural form.

• Il a des besoins importants. — He has important needs.
• Il y a un besoin urgent d'agir. — There is a pressing need to act.

Consider the difference between "I don't have to" and "I must not". The former expresses a lack of obligation, while the latter expresses an obligation to avoid an action. In French, to express a lack of obligation, use a negation with avoir besoin de or avoir à.

• Je n'ai pas besoin d'un stylo. — I don't need a pen.
• Nous n'avons pas besoin de votre permission ! — We don't need your permission!
• Elle n'a pas à parler. — She doesn't have to speak.
• On n'a pas à manger maintenant. — We don't have to eat right now.

To express "must not" in French, use a negation with devoir or il faut.

• Elle ne doit pas manger de poisson. — She must not eat fish.
• Nous ne devons pas nous mentir. — We must not lie to each other.
• Il ne faut pas réfrigérer les tomates. — One must not refrigerate tomatoes.
• Il ne faut jamais oublier les leçons de l'histoire. — We must never forget the lessons of history.

https://www.duolingo.com/skill/fr/Abstract-Objects

Quelque, quelqu'un & quelques-un(e)s

The indefinite adjective quelque ("some") can be combined with pronouns or nouns to create indefinite pronouns. For instance, chose means "thing", so quelque chose means "something".

• Nous écrivons quelque chose. — We are writing something.
• Je veux manger quelque chose. — I want to eat something.

Quelque can combine and elide with un ("one") to give quelqu'un ("someone"), which is singular.

• Quelqu'un est ici. — Someone is here.
• Je connais quelqu'un au restaurant. — I know someone at the restaurant.

For multiple people or things, use the plural forms quelques-uns (masc) and quelques-unes (fem), which are normally translated as "a few", or perhaps "some".

• Ce sont quelques-uns de nos meilleurs amis. — These are a few of our best friends.
• Quelques-unes de ces questions sont difficiles. — Some of these questions are difficult.

While quelqu'un only refers to people, quelques-un(e)s can refer to anything.

https://www.duolingo.com/skill/fr/Pronouns-2

Y & Lui

For most verbs, personal indirect object pronouns like lui can only refer to people or animals, but you can use the adverbial pronoun y for inanimate things.

• Elle ressemble à sa mère. ⇒ Elle lui ressemble. — She resembles her.
• Ça ressemble à un robot. ⇒ Ça y ressemble. — It resembles it.

Some verbs allow personal pronouns like lui to be used with anything you can personify. These verbs are dire à, demander à, donner à, parler à, téléphoner à, and ressembler à.

• L’enfant parle à son jouet. ⇒ L’enfant lui parle.
• Je demande un renseignement à la banque. ⇒ Je lui demande un renseignement.

Some French expressions don't allow any preceding indirect objects, notably être à, faire attention à, s’habituer à, penser à, revenir à, and tenir à.

• Tu fais attention à elle. (Not Tu lui fais...) – You are paying attention to her.
• Il pense à elle. (Not Il lui pense...) – He thinks of her.

Remember that y can also refer to locations.

• J'y vais. — I'm going there.
• Il y était. — He was there.

https://www.duolingo.com/skill/fr/Pronouns-2

Faire vs Rendre

In "Verbs: Present 1", you learned about the causative faire, which can indicate that the subject has directed someone to perform an action. Notice that faire is followed by an infinitive here.

• Je le fais réparer. — I am having it fixed.
• Elle lui a fait perdre 5 kilos. — She made him/her/it lose 5 kilos.
• Je leur ai fait faire de l’exercice. — I made them (get some) exercise.

The verb rendre ("to make") can also indicate that the subject has caused something to happen, but it's used with adjectives instead of verbs.

• Elle le rend heureux. — She makes him happy.
• Ça me rend fou ! — That drives me crazy!
• L'erreur a rendu le texte incomprehensible. — The error rendered the text incomprehensible.

https://www.duolingo.com/skill/fr/Verbs:-Infinitive-2

Stands for something

Sitesurf 

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"n'importe quoi" is short of "il n'importe pas quoi", meaning "it does not matter what".

So the n' is a negative.

https://www.duolingo.com/comment/6389254

Sitesurf 

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je dois en acheter un = je dois acheter un des chapeaux (one of the hats)

As I said, "en" can replace a singular (uncountable) or a plural:

• il y a de la viande; je dois en acheter (some)
• il y a des carottes; je dois en acheter (several)
• il y a des chapeaux; je dois en acheter un (one of them)

https://www.duolingo.com/comment/344650

vouvoyer = address as vous, speak formally
tutoyer = be informal, be on familiar terms
https://www.duolingo.com/skill/fr/Verbs:-Infinitive-2

subjunctive

BampaOwl 

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The six basic endings of the present subjunctive are -e, -es, -e, -ions, -iez, -ent. These are added to a stem which is derived as follows; take the 3rd person plural present indicative, remove the "ent" from the end, then add endings as above.So for boire, you take boivent, so the stem is boiv-, so the subjunctive is je boive.

BUT, quoting from my grammar book, "If the stem of the Present Indicative changes during the tense, the stem of the Present Subjunctive usuallyfollows suit. So for boire, the Present Indicative is bois, bois, boit, buvons, buvez, boivent, and the Present Subjunctive is boive, boives, boive,buvions, buviez, boivent.

Avoir and être are different, as you might expect. And then there's the Imperfect Subjunctive ...

Sitesurf wrote:
... "aimer que" which indeed drives the subjunctive mood in the subordinate clause, as well as the other appreciation verbs: apprécier, adorer, détester, haïr, préférer.

https://www.duolingo.com/comment/16040381$comment_id=16070931

Preposition + infintive

Pour generally marks the purpose, end, or object of an action. Can be translated to "in order to."

For c'est and il est, if it's a dummy subject:
impersonal expression + de + intransitive infinitive
or
impersonal expression + de + transitive infinitive + direct object

If it's a real subject:
impersonal expression + à + intransitive infinitive

http://french.about.com/od/expressions/a/impersonal.htm


the subject in the impersonal construction il est + adjective + de must be a dummy subject. If it's a real subject, you must use à instead of de.
https://www.duolingo.com/skill/fr/Verbs:-Infinitive-2

Passive infinitive follows the form: noun + à + infinitive

http://french.about.com/od/grammar/a/passiveinf.htm

These have different meanings depending on which preposition they use:

http://french.about.com/library/prepositions/bl_prep_a_vs_de2.htm

Nombres

Notice the et in the middle of 21, 31, 41, 51, 61, and 71.

21 = vingt-et-un
31 = trente-et-un
41 = quarante-et-un
51 = cinquante-et-un
61 = soixante-et-un
71 = soixante-et-onze
80 = quatre-vingts - notice the s at the end.

81 and 91 don't have et:

81 = quatre-vingt-un
91 = quatre-vingt-onze

In Swiss French:
70 = septante
80 = octante
90 = nonante

Notice that multiples of 100 have an ending -s, but there is no ending -s if cent is followed by another number.

144 = cent quarante-quatre

200 = deux cents
233 = deux cent trente-trois

Mille is never pluralized. Million (million) and milliard (billion) do pluralize, and they keep their ending -s even when followed by other numbers. Also, unlike cent and mille, million and milliard must be preceded by a number.

1 000 000 000 = un milliard

De must appear before nouns for million and milliard.

J'ai un million d'amis

Notes taken from https://www.duolingo.com/skill/fr/Numbers-2