How do you know which endings you should use in spanish for different tenses? er, ir, ar?

Verbs with -er, -ir, and -ar endings are generally infinitive verbs which one must conjugate according to a subject. How those infinitives are conjugated is dependent upon the subject's gender and plurality or singularity.

Take the infinitive seguir (meaning "to follow"). If you have the feminine singular subject ella, the verb seguir must take on a singular form; present tense would be sigue. Different tenses have different conjugations. In most cases, you just need to memorize the different conjugations for gender and tense.

Ellos  están jugando  el tenis.