Past

The Chinese language doesn't have a tense as we might know it from other languages. However, the Chinese people tend to rely on a time adverb or an aspect particle to the sentence to indicate the "past, present, and future tense."

Unlike in English and other European languages, the form of a Chinese verb remains the same, regardless of whether it is past, present, or future tense.

Add the particle "了" and "过" after the verb to express completed actions. 了 is used to indicate the completion of an action. Similarly, 过 is used to show that something has happened before. For example:
1.我去过北京( I have been to Beijing)
2.我吃了一个鸡蛋(I ate an egg)

But "了" is a complicated particle: it is normally not attached to stative verbs such as 是 and 在. (有 is different — it takes 了 when it marks a change of state, e.g. 他有了新工作 “he has got a new job”.)
Also, the particle 了 can also be used for the present or the immediate future, so be careful of its usage and be sure to understand both functions.
他病了 He is sick
我要走了 I will leave soon

Another simplest and clearest way for you to indicate that an action occurred in the past is by using commonly used time phrase or adverb, see below:
以前(before), 过去(in the past), 上周(last week), 昨天(yesterday), 去年(last year), 刚刚(just now)