Present Perfect is

a ) to indicate a completed activities in the immediate past :

eg He has just gone out.

b ) to express past actions whose time is not given and not definite :

eg Mary has been to Japan.

c ) to denote an action beginning at some time in the past and continuing up to the present moment

eg We have lived here for ten years.

Past Perfect is two actions took place in the past. One is earlier than the other.

eg When I came in, you had gone

I think there is no need to explain the continuous form. For the word ' then ', it means a particular time in the past or future.

eg The Vietnam war broke out during the 70s, it was the then president of the US decided to commit their troops to it.

eg I'll see you then ( a particular time in the near future