well, you just for the subject and the adverbs that let you know what is on the point and whether it is in the present, future or past.
I am not sure an example would be of great help, however here it goes on very simple saying:
我上学: I am going to school
昨天我上学: yesterday I went to school
Notice that the three last signs are just the same, you just have to add 昨天 (which means yesterday) ahead of the sentence to show that the action actually took place yesterday.
hope that helps
(and hope my chinese is correct in this post
)