Affirmative:

Subject + Aux.Verb + V-ing + Compl.

• We are playing tennis now
• Luis is doing his homework 

Negative:

Suject + Aux.Verb + Not + V-ing + Compl.

• We are not playing tennis now 
• Luis isn't doing his homework

Interrogative:

Yes/No Question

Aux.Verb + Suject + V-ing + Compl.?

• Is luis doing his homework right now?

       Yes: -He is

       Yes: -Luis is doing his homework...

        No: -He isn't

        No: -He isn't doing his homework...  

Inf. Question

W.H. + Aux.Verb + Subject + V-ing + Compl.?

• What is luis doing right now?
• Where is he doing his homework?