下午好,我今天来给大家讲第16课.用英语怎么说.

这个句子的中文虽然很简单,但是对应的英文表达方式可以有很多,给你几个供参考：
1、Good afternoon,today,let's studing 16th classes.
2、Hi everyone.wellcome to learn 16th,today.
3、Good noon,I will to explain to you the 16th class today.
4、Good afternoon,I want to speak to you the 16th class today

good after, today i would talk about lesson 16