What time to use with while?

I'm not sure what time I should use after while in this sentence:

He constantly thinks about their correspondence while he was in prison.

Solution 1:

As tchrist pointed out in a comment, your sentence calls for the use of some expression like "from while" in place of "while".

The word while by itself is generally a synonym for "during", and is used to express events that happened at the same time (or an event that happened during a certain period of time). For example, you could say

He constantly thought about their correspondence while he was in prison.

to express the idea "During his time at prison, he constantly thought about their correspondence."

He constantly thinks about their correspondence while he was in prison.

This sentence, as written, sounds incorrect.

To express the idea "Now that he is out of prison, he constantly thinks back to their correspondence from the time when he was in prison", you can say (as tchrist mentioned):

He constantly thinks about their correspondence from (back) while he was in prison.

(the word "back" is optional)

Since he is thinking at a time when he has finished with being in prison, you could use the word "when" instead:

He constantly thinks about their correspondence from (back) when he was in prison.

It actually sounds most natural to me to use "from when he".