I think there are four main forms to use past tense. These just popped in my head, I did't spend much time doing a research about it, and it is not from grammar book. A. Verb+了 我吃了饭。/ 我去了一趟办公室。 B. Verb+过 我看过那一场电影。/ 我去过那个公园。 C. Verb+过+了 1. 我去过了超市。/ 我吃过了饭。 2. 我去过超市了。/ 我吃过饭了。 D. 代表过去时间的副词，比如，曾经、原来、刚刚、先前...+Verb 他曾经是一名律师。（现在不是了） 他原来喜欢跳舞，妻子去世后就再也不跳了。 我刚刚去楼下买一盒巧克力。 他先前就知道事情的真相，只是不想而已。

adding words of past time: I was your friend = 那 时候 我 是 你的 朋友。 he was a doctor = 以前 他 是 医生。 Kathmandu was a lake. = 先期 加德满都 是 一个 湖。

Chinese does not present 'time' by changing the form of verbs, that is, it does not have a morphology dealing with 'tense'. Chinese carries the meaning of time usually by 2 ways. 1) by the context 2)using words with meaning of past time, for instance, 昨天，几年以前 Also, some adverbs ——as given by youkoufiona ——contain the meaning of past time will help put it. But the first 3 ways that youkoufiona had given are probably not right. They do not denoting past time actually. —— I'm sorry, youkoufiona, your answer was great teaching material for the usage of 着，了，过 , but not a very good one for the very question. I'm not bashing you, I only do not want the Chinese learners to be misled or confused. I'll give some counter-examples so as to make it clear. On the auxiliaries 着，了 and 过 there have been a lot of great research and believable results. -------------- I think there are four main forms to use past tense. These just popped in my head, I did't spend much time doing a research about it, and it is not from grammar book. A. Verb+了 我吃了饭。/ 我去了一趟办公室。 ------ C.E. 孩儿他爸，你明天吃了早饭就去镇上买豆油啊！ B. Verb+过 我看过那一场电影。/ 我去过那个公园。 ------- C.E. 孩儿他爸，你明天吃过早饭，不要耽搁，赶紧去买豆油啊！ C. Verb+过+了 1. 我去过了超市。/ 我吃过了饭。 -------- 将来你飞机也做过了，轮船也做过了，就不觉得它们稀奇了。 2. 我去过超市了。/ 我吃过饭了。 D. 代表过去时间的副词，比如，曾经、原来、刚刚、先前...+Verb 他曾经是一名律师。（现在不是了） 他原来喜欢跳舞，妻子去世后就再也不跳了。 我刚刚去楼下买一盒巧克力。 他先前就知道事情的真相，只是不想而已。