Many people speak French fluently but still run afoul of little grammatical details.  A typical example is the proper use of the preposition pour.  If I had a dime for every time I’ve heard English-speakers say in French chercher pour un travail or attendre pour quelqu’un, I would be rich. Please, spare the ears (don’t écorcher les oreilles) of French-speakers and remember that the verbs chercher and attendre do not require pour. So, you would say:

Je cherche un travail. (I’m looking for a job.)
Mon frère attend son amie. (My brother is waiting for his girlfriend.)

This kind of mistake can be insidious because it becomes ingrained in one’s speech and will be very hard to eradicate if left uncorrected.

Although pour can be often translated by “for” and vice versa, there are a couple of major differences between English and French that I would like to point out.

Merci de or Merci pour followed by a noun

In English, when we thank somebody for something, we use constructions like: “thanks for your help.” or “thank you for the suggestion.” The temptation here in French is to say: merci pour votre aide or merci pour la suggestion.

This sort of construction is totally understandable and not really bad.  Current French usage is a bit vague here, but one thing is very clear,  merci de is more sophisticated and is preferable. So, you can’t go wrong with the following constructions:

merci de votre aide (thanks for your help)
merci de votre suggestion (thanks for your suggestion)
merci de votre attention (thanks for your attention)
merci de votre confiance (thanks for your confidence)
merci de votre appui (thanks for you support)
merci de tes mots gentils  (thanks for your kind words)

Merci de or Merci pour followed by a verb

If you want to say something like “thanks for coming” or “thanks for having me,” this is where the merci pour will get you into trouble. Look at the following examples:

merci de m’accompagner (thanks for accompanying me)
merci de venir m’aider (thanks for coming to help me)
merci d’avoir été là (thanks for being/having been there)
merci d’être venu en si grand nombre (thanks for coming in such large number)
merci de t’être levé de si bonne heure (thanks for getting up so early)

Notice how merci de is followed by the infinitive form including the past infinitive (avoir été). Do not use merci pour in this situation.

Expressing time or duration with pour

The other day I heard someone say:

Je suis à Montréal pour deux semaines (I’m in Montreal for two weeks)

I assumed that the person was going to stay in Montreal for two weeks. But after some discussion I realized that the person had actually been in Montreal for the past two weeks and was leaving the following day.  What the person should have said was:

Je suis à Montréal depuis deux semaines. (I have been in Montreal for two weeks.)

Notice how the “je suis” is this case is translated as “I have been.”

Mistakes with prepositions are common and often quite subtle.  Just keep your eyes and ears open. And remember, always check with a native speaker if you have any doubt.