Replying to Some grammar help please (+K for help)

just had like 50 things to fix on a sheet of grammar mistakes, and i cant get these two. im sure the mistake is pretty obvious, but im not that great at grammar so im having trouble with these last two.

A revolution is when the government changes hands as a result of a violent uprising.

and

He often forgets his office key at home.

Thanks (and + K for all who help)