Statements or questions concerning actions or non-actions in the past usually require the use of “le” and “mei” when in the negative. As in

• 昨天我看了一个电影。
• 去年她在中国工作了一年。
• 上个星期，他因为生病没来上课。

However, there are many exceptions to this rule, and you should not think that “le” functions like suffix “ed” for many English verbs to indicate past actions. If and when the stress and emphasis is other than the action itself, the nuance becomes shifted and “le” is often omitted, as in

• 昨晚我12点才睡觉。I didn’t sleep last night until midnight.
• 跟他住在一起的那些日子，我一点也不幸福。I was not happy in the least in those days when I lived with him.
• 去年我搬家四次；他每次都来帮我的忙。I moved four times last year and he came to help me move every time.
• 因为没车，今天上午我只好走路去看我的朋友。Without a car, I had to go see my friends on foot this morning.
• 小时候，他姐姐从来不跟我们玩。When little, his older sister never played with us.
• 爸爸如果活着，不会这么说。If Dad were alive, he would not say something like this.
• 我上个星期就知道你不喜欢她。I knew it last week that you didn’t like her.
• 昨天不给你打电话是因为怕你在睡觉。I didn’t call you yesterday because I was afraid you were asleep.