The verb in Chinese has no tense variety.“了”in the end of the sentence means you have allready done something or you are going to do it right now. "Jin tian ni zuo shi me le?" in this sentence,le means to sent out a question. le is one of the hardest part for foreigner to learn.

It's an interesting question, it's hard to answer without examples. Let me tell the difference. “Jin tian ni zuo shen me” means what are you going to do today. “Jin tian ni zuo shen me le” means how did you spend your day today. “ni zuo shen me ne” means what are you doing now. But “le” is not always used in past tense, we use it to emphasize the result, effect or influence of the action. Is this more clear?

"le"a common phrases, such as "wo yao shui le "

动态助词，放在动词和宾语的中间，用来表示已经结束的事情，例如：我看了你的问题。 现在你明白了吗？

"le"often doesn't mean anything,it just a interjection,like subjunctive in english.but it makes stentence more complete,use it in the end of sentence,so it is important.