Avoir auxiliary
Avoir in compound tenses
There are only two auxiliaries in French: avoir and être.
Avoir is mainly used in compound tenses: j'ai mangé, il aurait su...
To build present perfect or past, you have to take the avoir auxiliary and add verb past participle.
Avoir, for action verbs
Avoir is used to conjugate verbs which express an action. For status verbs, you have to use the other auxiliary être.
Even if the large majority of verbs are action verbs, most of them can be conjugated with the two auxiliaries. Of course, there meaning is changing...
With avoir, verbs express an action meanwhile they express the result of this action with être.
aborder crever expirer
descendre changer enlaidir
rajeunir vieillir maigrir
...
Eg: J'ai descendu le piano.
Je suis descendu.
This list is not complete and there are lots of verbs of this kind.