The infinitive is one of the impersonal forms of the verb and does not conjugate. Its form always stays the same irrespective of gender and number.
There is the present infinitive and the past infinitive. The present infinitive regular verbs finish with -ARE, -ERE and IRE.
The past infinitive is used to talk about a finished action. To make the past infinitive, we use either the verb avere or the verb essere.
Oggi devo lavorare fino alle 20:00. (present infinitive)
Today I have to work until 8 pm.
Dopo aver pulito, ho guardato la televisione. (past infinitive)
After cleaning, I watched the television.
Dopo essere stato in vacanza per tre mesi sono tornato a scuola. (past infinitive)
After being on holiday for 3 months, I went back to school.