you use it to talk about a man who is currently sleeping and not present :) 

هو يدرس he study - present 

انا ادرس i study  - present 

انا درست its the past in arabic ...i studied 

if you want to use the verb in The Imperative it will have the same letters of the present ' Adrusu أَدرس انا i study . but the difference is that u have to put Damma on the Alif Udrus Anta أُدرُس 

And so on with the verb ينام  to sleep.. only one difference with the Imperative of this verb that is irregular will be ' Nim نِم

Good luck