Question

He was upset because somebody(steal)his watch

Answer 1

Answer:

He was upset because somebody stole his watch.

Explanation:

It makes sense to use stole instead of steal!

It also would be better to use "past tense"

Answer 2

Answer:

He was upset because somebody 'stole' his watch

Explanation:

Subject verb agreement.

and also was is in the past tense so stole is the appropriate verb to use