My dad had brain stroke last year. tonight he is mumbling, not finishing a sentence. not saying what's the problem. is this a sign of stroke?

Likely: He may well have another stroke and i feel he should be examined by a physician asap and the necessary testing will be done.

This Could Be: If he is having trouble with speech it could be the early signs of a stroke or a transient ischemic attack. It would be good to seek medical attention for him right away.

Can be: Definitely, your dad should be taken to hospital right away for a brain scan. There can be other reasons too, but he needs immediate attention.