In advance, sorry for the tl;dr;.
My knowledge of characters is awful (sorry), but I can provide some grammar help with pinyin (I only have a year of Chinese under my belt).
Basically, tense is shown using the time. The particle "le" (δΊ†, the character you used) is the particle used to indicate that something in process is at an end. For instance, when saying "What time is it now?" [xianzai shi ji dian le?], the "le" is a vestige of the fact that the clock has reached the current point in time. The "le" particle can be a little confusing, but it mostly comes into play when talking about age or time.
I'll try to give some examples of past tense usage:
Zuotian(2,1) women(3,1) da(3) lan(2) qiu(2).
Yesterday we played basketball.
[Tones are marked in the parentheses. Again, sorry for me being awful with characters.]
For this, all I needed to say when the action happened. In this way, the idea of a basic preterite is formed (I did it yesterday, the action was completed). Ongoing action initiated in the past is not something I'm familiar with in Chinese (1 year doesn't get you very far, especially with a teacher who isn't very good).
This same idea can be applied to future events as well. "xing qi liu wan sheng wo hen mang" [saturday evening I am very busy]. "wanshang liu dian ban women chi wanfan" [we'll have dinner at 6:30PM]. Again, sorry about toneless pinyin (and no characters).
Hrm... that's a basic idea of it. The "le" particle is a little confusing (and I'll fully admit not knowing completely how and where it is used).
Just remember the general format:
[time] [subject] [verb] [modifier] [object] [particles]
Yeah...
Again, anyone with more experience, feel free to correct anything I just said.