Please let me know whether FINITE VERB means:the verb which changes according to the subject or is it wrong?
Thank you in advance

A finite verb has a tense and takes a subject. So the subject will not solely dictate the verb, the tense will too. E.g. 'Richard TRAINS hard' (present tense); and 'Richard TRAINED hard' (past tense).
I hope that helps a little.

but how can we identify them in presence of adjective

He is a nice boy. 'is' = finite verb, nice = adjective.

He killed a white bird. 'killed'= finite verb, white = adjective

He is a trained student. 'is' = finite verb, 'trained' = adjective.

Are you clear?

Here is a lesson (with a test) on finite verbs.