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

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

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