How do I form the regular past tense?

In Russian, regular verbs are formed by using the STEM of the infinitive and a SUFFIX, which agrees in both GENDER and NUMBER: Masc: -л / Fem: -ла / Neut: -ло / Plural: -ли The STEM is usually the infinitive - "ТЬ". For example: Думать - Дума - Думал (To think - thought). Therefore, if we were to agree this we have: Он думал - He thought / Она думала - She thought / Оно думало - It thought / Они думали - They thought