1,when u use 了after a verb,it means the action have been done. eg:吃了 means you have finished eating 2,when u use 了at the end of a sentence,it means there is a change&some new situation occurs. eg:外面下雨了。

了 has a number of uses, but its most general meaning is "the completion marker". You need to know what is being completed in order to place it. The best description of 了 I have seen is from FSI Mandarin Chinese course: http://fsi-language-courses.org/Content.php?page=Chinese Download the PDFs and read up the lessons on 了.

I think Mary gave a good explain. “了”1）after a verb,or a adjective,means the action or the change had finished or happened . means"already" or "is doing sth" eg.A:"Have you finished/eaten your breakfast" B:"Yes,finished/have eaten"= 吃chi1了 eg."The leaves have turned yellow", = yellow already=黄huang2 了。 eg. "She is crying"=她ta1 哭ku1 了。 2)快kuai4了= will finish soon. eg."Have you finished your breakfast?' "No,but will finish soon" 3)when no "了“after "吃chi1=eat”， eg.“你吃苹ping2果guo3吗？” 苹果=apple 吃=eat “would you like an apple?" "No, thanks"=不吃，谢谢。

you can refer to mary's answer. the first situation can be 了 after a verb or an adjective. and the second situation can be 了 at the end of a sentence or at the pause a sentence.

了means a lot! sometimes it means past tense sometimes it means in progress。。 it is complicated to explain