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I can't figure whether to use à or de with an infinitive to modify a noun or an adjective. I understand the impersonal construction ce+etre+adjective. À is used before the infiniitve when the grammatical subject is ce. But what about with a personal pronoun as the subject.
Here are just some of the sentences I have seen
Je suis pret à partir/ils ont été forcés de partir/Nous avons la permission d'entrer
It all seems so random. Are there rules which define which one to use in and, if so, what are the rules or are both interchangeable?
The answer is that it can be quite a complex issue and in some cases, the choice of "à" or "de" (or indeed neither) is what it is "just because that's what people use".
However, there are a few general patterns. Quoting myself from a previous thread:
No preposition between the verbs:
- so-called "modal" verbs (vouloir, devoir, pouvoir, savoir)
- verbs of stating (dire but also near synonyms like avouer, affirmer, croire/penser/considérer,prétendre...)
- verbs giving an opinion about the action of the second verb (préférer, aimer, détester, souhaiter)
- usually, verbs of motion (aller, monter...)
- a few special "causal" verbs that indicate the object of the first verb being forced to carry out the action of the second verb (laisser, faire, also envoyer)-- note that other verbs indicating "starting" of an action generally take à, and verbs of "commanding" take de
Preposition à used:
- most cases when there is a notion of starting the action of the second verb (apprendre à...,commencer à..., se mettre à..., conduire/mener/pousser/inciter qn à..., décider qn à..., se décider à..., viser à...)
- some cases where à would have been used with a noun anyway (with overlap with the previous subcategory), e.g. on l'a poussé au crime/à commettre des délits
Preposition de used
- many verbs of "commanding" or recommending, including commanding oneself:commander/demander/persuader/recommander/conseiller (à) qn de faire qch
- when there is a notion of "stopping" the action: arrêter, cesser, empêcher
- many cases where de would have been used with a noun, and where de carries a genuine value of "from", "of", "for": accuser/remercier/soupçonner qn d'avoir fait qch
However, in reality it's a bit more complex than this, and as you get more fluent in French, you'll pick up some of the nuances and exceptions to the above.
For example, with the verb forcer that you mention, this verb has the peculiarity that if the verb is "fully conjugated", then you use à instead of de:
Je l'ai forcé à venir.
and indeed, if an agent is involved in your sentence, many speakers would change the de to à:
Ils ont été forcés à partir par les autorités.
Now, this is not to get you panicked: if you start with the above general rules I mention you'll be well on the way. But it's just to point out that you need to be on your guard for complexities/exceptions!
The last one sounds just a bit strange. Not to say that nobody would use it, but I'm pretty sure I'd say Ils ont été forcés de partir par les autorités, even though I use your other example (then again, I live in Switzerland, not Paris).
So, I was wondering if it has something to do with the fact that, tough there is an agent, it's not the subject in that sentence. Any idea on that?
That certainly has an influence: usually with a passive, the whole point of using the passive is often that the agent isn't mentioned (ils ont été forcés de partir). And in such cases, de is practically always used[*].
So when you introduce an agent, there's a kind of conflict between the "normal" active verb, where à would tend to be used, and a passive with an agent. And yes, there is variation in usage in this case. I thought the use with à (as in my example) was still reasonably common. But I suppose it wouldn't surprise me to learn that in fact de was now more common.
I wasn't aware of a regional difference on this. But it's the kind of thing that could be subject to regional differences, I suppose.
[*] I say "practically" because there are corner cases where you will find authors using e.g. Il le força d'attaquer to avoid having three "a" sounds in a row (compare il le força à attaquer).
Thanks for the answers. Does a passive nuance also influence infinitives when they modify nouns ? So, for instance, if something was being done to/with the noun you would use à+infinitive:Il y a un appartement à louer. Whereas if the noun was not undegoing the action you would use de:Nous avons la permission d'entrer.