Difference between a and à
The words a and à are grammatical homophones, i.e. they do not have the same grammatical function in the sentence.
- a comes from verb avoir conjugated to the indicative present : il a.
- à is a preposition.
The best way to do their distinction is to change the sentence to another tense such as imperfect. At imperfect, a becomes avait meanwhile à does not change.
We always write à in front of an infinitive and a in front of a past participle. To recognise the infinitive from a past participle, you can take a third group verb such as prendre, vendre, dormir... In addition you get the gender: pris, prise or prendre.
- il a dit de ne pas s'en faire. (present)
- il avait dit de ne pas s'en faire. (imperfect)
- il ne pense qu'à manger. (Replace by another veb)
- il ne pense qu'à dormir.
You always write "a"in front of an infinitive and "a" in front of a past participle. To recognize the past participle of the infinitive, we can take a verb from the third group like "prendre" (to take), "vendre" (to sell), "dormir" (to sleep)... In addition, we get the agreement: pris, prise or prendre.
As for "as", it is simply the verb to "avoir" conjugated with you: "tu as". Notice the "s" at the end which is the mark of the 2nd person. A simple way to recognize it is to try to replace it with "nous" and put "avons" and find the mark of the "tu". If so, then it is "as".
- tu as pu venir nous rendre visite. ==> nous avons pu venir.
- tu as une veste beige. ==> nous avons une veste beige.
- We use "a" when it is verb "avoir". If at past, it is possible to say "avait", then it is "a".
- "à" is the preposition, not the verb. In the past tense, you can't put "avait".
- "as" is verb "avoir" conjugated with "tu".