Some verbs are used extensively as Auxiliaries, to help other verbs to build up Compound Tenses. They lose their own meaning in the process. In French, there are two principal auxiliary verbs, être and avoir. Because of their use as Auxiliaries, they are by far the most common verbs in the language. (The five most common words in our Frequency List are, in order: de, le, être, un and avoir.)
For a different sort of auxiliary, check out the Causative construction.
To read anything in French, you must be familiar with the present,
imperfect, future and conditional tenses of être and
avoir, given here accompanied by personal pronouns (je,
tu, il... = I, you, he...).
The most obvious difference between English and French is that French has two Auxiliaries to form past tenses while English only has one. In French, some very common verbs use être as an auxiliary instead of avoir, and this can be confusing. You say Elle a commencé (she began, or she has begun), but Elle est arrivée (she arrived, or she has arrived).
You must be careful not to confuse these compound forms with être with continuous forms in English (she is arriving = elle arrive).
Il est devenu plus pâle
He got paler (not he is getting, which would be simply il devient - present tense)
Elles sont arrivées les premières.
They arrived first (not they are arriving = elles