This is an irregular verb. It follows the conjugation pattern of the verb chegar.
Present:
A Ana rasga o envelope com cuidado.
Ana tears the envelope carefully.
Preterite:
O Miguel rasgou a camisa na cerca.
O Miguel rasgou a camisa na vedação.
Miguel tore his shirt on the fence.
Conditional:
A Clara rasgaria a carta sem ler.
A Clara rasgaria a carta sem a ler.
Clara would tear the letter without reading it.