Affirmative:

Subject + Aux.Verb + V-ing + Compl.

• We were playing tennis yesterday morning 
• Luis was doing his homework at home last night

Negative:

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

• We were not playing tennis yesterday
• Luis was not doing his homework at home...

Interrogative:

Yes/No Question

Aux.Verb + Subject + V-ing + Compl.?

• Were we playing tennis yesterday?

      Yes: -We were 

      Yes: -We were playing tennis...

      No: -We weren't

      No: -We weren't playing tennis 

Inf. Question

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

• What were we playing yesterday?
• Who was playing tennis?
• when were we playing tennis?