Harmeet, there is no past tense for verbs in Mandarin. We use time words to indicate the past tense. Besides, some words like “le”, “guo” also indicate the past tense. For example:

Yesterday was raining = zuo tian xia yu le

In this phrase, we use “le”, which means “xia yu” already happened.

Another example:

I was a teacher. We say “wo yi qian shi yi ge lao shi”. In this sentence, “yi qian” is a time word, means “in the past”.