Using 了
了 marks an event as complete, it can refer to an event in the past, the present or the future. It appears after the main verb. 了 can also be used as a sentence level particle indicating a change of state. It appears at the end of the sentence.
Examples:
他回家了我们就吃晚饭.
Tā huí jiā le, wǒmen jiù chī wǎnfàn.
After he came home, we had supper. / After he comes home, we will have supper.
There are a number of expressions that commonly appear with le:
太好了 tài hǎo le excellent!
对了 duì le correct!
说定了 shuō dìng le that's settled!