Depuis vs Pour

Question

To say - "The thief is in prison for 2 years." ....are all of the below correct/acceptable ? Is 'Depuis' more preferable with present tense/ present durations and no. 2) 'Pour' is incorrect? because Pour is used with future tense/ future durations only as per this lesson?? Please clarify.
 
1. Le voleur est en prison depuis 2 ans. 
2. Le voleur est en prison pour 2 ans. 
3. Le voleur est en prison pendant/durant 2 ans. (also acceptable bcoz action is within a specific timeframe, as per this lesson?)
 
So, out of the above no. 1)- 'Depuis' is the most preferable way to say it because we are using present tense (and the action is still ongoing)?? And 'Pour' is not correct because it is strictly for future durations as per this lesson?

Accepted Answer

All three sentences can be correct, but they convey slightly different meanings:
Le voleur est en prison depuis 2 ans. -- The thief has been in prison for 2 years. It emphasizes the duration of time the thief has already spent in prison. Le voleur est en prison pour 2 ans. -- The thief is in prison for 2 years. It emphasizes the predetermined duration of the thief's prison sentence.
Le voleur est en prison pendant/durant 2 ans. -- The thief is in prison for 2 years and emphasizes the continuous duration of the prison stay. "Pendant" and "durant" are interchangeable in this context.