AFTER vs THEN

 AFTER

""After" can be:

1. PREPOSITION: very common use. AFTER + NOUN (always!)

  We will meet after lunch

  I will go after you 

2. CONJUNCTION: AFTER + sentence

We can go to the beach after we have finished our homework

THEN

It's an adverb. It connects one sentence with the previous one.

They spent two hours working and then they went to the restaurant

If you wish to use AFTER you have to write AFTER THAT

They spent two hours working and after that they went to the restaurant