Jada explained pretty much what you asked. Just one thing she missed: there are different implied meanings between "我昨天见过他" and "我昨天见了他". "我昨天见过他" can imply two meanings: 1. You did not plan to see him yesterday, but came across him on the street or somewhere. 2. You are surprised to see him again because you met him somewhere yesterday, especially when you use " ！", which is an emphasis on your surprise. Here is a scenario: James saw a really pretty girl on bus yesterday, and she had an impression on him. He felt sorry he did not ask her number. Now at the bar he is surprised to see her again sitting next to him! James: 我昨天见过你！ The girl: 是吗，我还见过你爸呢！ "我昨天见了他" implies that you planned to see him, and you saw him yesterday. Here is a scenario: Chris: Hey! James, how did your wedding go yesterday? Did you see your wife's ex? I bet he came. James: 我昨天见了他。还是那么挫。 Hope it could help you;)

I am really happy that I can help you, I hope that you can make a great progress on your Chinese. Best wishes.

“了” meas something happened (past tense) or has already happened (past ferfect tense) “过” means something has already happened and was over (past perfect tense) See the difference: 他上个月去了北京。(Perhaps he is still in Beijing now.) 他上个月去过北京。(He has come back.) Actually most of the time they are interchangeable in past ferfect tense, and they are always been put together, e.g., 警察来过（了）(They were gone)；下过雨（了）（It stopped）；我吃过饭了；我看过《蜘蛛侠》了。Just a way to emphasize something has already happend or been done. But if you use "了" without "过", the meaning of a sentence will depend on the context. e.g.,我吃饭了 can mean I already had a meal or I am going to have a meal in oral Mandarian. So it's safe to use "过" when you want to say something has happened or has been done.

Thanks Crystal! Yes that definitely makes sense.. Now I just have to remember to apply this when I am constructing sentences in conversation!!! That's a whole other issue! :)

In Chinese, we often add words like ‘昨天’ or '以前‘ to express that an action happened in the past. We always use '已经' and '过‘ together to express that we have completed an action. For example, if you want to say ' I have already finished my work' it means ' 我已经完成了我的工作’. And the sentences you mentioned above ' 我昨天见过他‘ and ’ 我昨天见了他‘ represent the similar meaning in Chinese but the tiny difference is that we often use 过 in this way '我昨天(已经)见过他’ so it means a completed action “ I have already seen him' and it contain a meaning that maybe you don`t need to see him today. While when you say '我昨天见了他’, it just means 'you saw him yesterday' and it does not stress whether you need to meet him today or not. I hope my comment can help you understand it better.