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

Verbs with different meanings with à and de

Verbs with different meanings with à and de

décider à		to persuade, convince
décider de		to decide to
demander à		to ask (for permission)
demander de		to ask (s.o. to do s.t.*)
jouer à		to play a game or sport
jouer de		to play an instrument
manquer à		to miss someone
manquer de		to neglect (to do s.t.)
(more about manquer)
parler à		to talk to
parler de		to talk about
penser à		to think about (imagine)
penser de		to think about (opinion)
(more about penser)
profiter à		to benefit
profiter de		to make the most of
venir à		to happen to
venir de		to have just (done s.t.)
(more about venir)

 Verbs that use both à and de in the same sentence

conseiller à qqun de faire qqch		advise s.o. to do s.t.
défendre à qqun de faire qqch		forbid s.o. to do s.t.
demander à qqun de faire qqch		ask s.o. to do s.t.
dire à qqun de faire qqch		tell s.o. to do s.t.
interdire à qqun de faire qqch		forbid s.o. to do s.t.
ordonner à qqun de faire qqch		order s.o. to do s.t.
permettre à qqun de faire qqch		allow s.o. to do s.t.
promettre à qqun de faire qqch		promise s.o. to do s.t.
téléphoner à qqun de faire qqch		call s.o. to do s.t.

qqun	quelqu'un		s.o.	someone
qqch	quelque chose		s.t.	something

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

Noun + de + noun (no article)

A "noun of noun" case happens when the second noun gives some information on the first noun's content, material or purpose:

• content: une bouteille de lait = a bottle of milk
• material: un pont de pierre = a stone bridge
• purpose: un couteau de cuisine = a kitchen knife.

As you can see, this French construction does not exactly match English where a noun can become an adjective if you place it in front of another noun. In French, you cannot use a noun as an adjective.

Written by Sitesurf@https://www.duolingo.com/comment/14079873$comment_id=14082490

de/des, neuf/neuve & nouveau/nouvelle

RoniG 

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Why is it "de" and not "des" if the items are plural?

Sitesurf 

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Because the adjective is in front of the noun.

• "des recettes nouvelles" but "de nouvelles recettes"

In front of an adjective, the indefinite, plural article "des" becomes "de".

Johnny446750 

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How come it's not neuves? Since she writes (not copies) the recipes.

Sitesurf 

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Something "neuf/neuve" is just out of the factory, so "une recette" will not be described as "neuve".

Johnny446750 

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Hmm, if a writer is writing a book, not published yet, is he writing un neuf livre or un nouveau livre? It sounds like a writer writes un nouveau livre but a publisher publishes un neuf livre (first edition). Is that correct?

Sitesurf 

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"Un nouveau livre", for the same reason. The making of the book is not so much in printing it but rather in writing it.

So, yes, "un livre neuf" (never used/read) will be just out of the printing plant, even if it is the 12th edition and/or the writer died 2 centuries ago.

https://www.duolingo.com/comment/211316$comment_id=13756219

quoi

Sitesurf 

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"quoi" never starts a question.

It is used either with a preposition: à quoi, de quoi, par quoi...

or as a direct object in informal questions: elle lit quoi ?

https://www.duolingo.com/comment/13676043$comment_id=13711492

Verb + de + infinitive

The French preposition de is required after certain verbs and phrases when they are followed by an infinitive. Note that the English translation may take an infinitive (to agree to do something) or a gerund (to be afraid of flying).

• accepter de: to accept, agree to
• achever de: to finish ___-ing
• accuser (quelqu'un) de: to accuse (someone) of
• s'agir de: to be a question of ___-ing
• (s')arrêter de: to stop ___-ing
• avertir (qqun) de: (ne pas) to warn (someone) (not) to
• avoir peur de: to be afraid of ___-ing
• blâmer (qqun) de: to blame (someone) for ___-ing
• cesser de: to stop, cease ___-ing
• choisir de: to choose to
• commander (à qqun) de: to order (someone) to
• conseiller de: to advise to
• se contenter de: to be happy ___-ing
• continuer de: to continue ___-ing
• convenir de: to agree to
• craindre de: to fear ___-ing
• décider de: to decide to
• défendre (à quelqu'un) de: to forbid sbd to do sth
• demander (à quelqu'un) de: to ask sbd to do sth
• se dépêcher de: to hurry to
• déranger quelqu'un de: to bother sbd to
• dire (à quelqu'un) de: to tell sbd to do sth
• s'efforcer de: to endeavor to
• empêcher de: to prevent, keep from ___-ing
• s'empresser de: to hurry to
• ennuyer quelqu'un de: to bother/upset sbd to
• essayer de: to try to
• s'excuser de: to apologize for ___-ing
• féliciter de: to congratulate for ___-ing
• finir de: to finish ___-ing
• gronder de: to scold for ___-ing
• se hâter de: to hurry to
• manquer de: to neglect, fail to
• mériter de: to deserve to
• offrir de: to offer to
• oublier de: to forget to
• (se) permettre de: to allow (oneself) to
• persuader de: to persuade to
• prendre garde de: to be careful not to
• prendre le parti de: to decide to
• se presser de: to hurry to
• prier de: to beg to
• promettre de: to promise to
• proposer de: to suggest ___-ing
• refuser de: to refuse to
• regretter de: to regret ___-ing
• remercier de: to thank for ___-ing
• rêver de: to dream of ___-ing
• risquer de: to risk ___-ing
• se soucier de: to care about ___-ing
• se souvenir de: to remember ___-ing
• supplier de: to be / beseech
• tâcher de: to try to
• venir de (faire quelque chose): to have just (done sth)

Written by NoriegaE@https://www.duolingo.com/comment/242649

demander à vs. demander de

• demander à manger quelque chose (the one who will eat is the same as the one asking for something to eat)
• demander de faire quelque chose (he one doing something is not the person asking for something to be done)

Written by Sitesurf@https://www.duolingo.com/comment/242649

essayer de vs. s'essayer à

Question: Is there ever a case where "Tu essaies à..." or is it always "Tu essaies de..."? 

Sitesurf 

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In its non-reflexive form, "essayer de + infinitive" is the only correct construction.

However, "s'essayer à quelque chose" or "s'essayer à faire quelque chose" means: to try one's hand at something/doing something.

https://www.duolingo.com/comment/242649$comment_id=13628101