The sentence is formed of three parts:
- Person eg. Elle (She)
- Avoir/être conjugated with the person eg. Elle a (She has)
- Past Particle eg. Elle a mangé (She has eaten)
Each verb is associated either with être (to be) or with avoir (to have). Most verbs go with avoir, those which go with être tend to be associated with movement eg. aller (to go), tomber (to fall) or even mourir (to die, moving into a different state).
The past particle changes depending on the ending of the verb.
er becomes é eg. mangé (ate)
ir becomes i eg. fini (finished)
re becomes u eg. vendu (sold)
It is also really important to learn irregular verbs and their past particles.