What's the difference between يتكلموا and يتكلمون? thank you!

How would I know when to use each one?
Jazak allahu kharan

Also when can I use the word بلى, does it mean "yea"?

يتكلمون = present tense =  They speak

يتكلموا  = It will not be used alone in a sentence.

When   negative prepositions  like   لم  or   لن    enter in this verb  يتكلمون  it will become

لم يتكلموا   or  لن يتكلموا -   The  letter   ن   will be dropped.

لم يتكلموا  =  They didn't talk  or  They didn't speak

لن يتكلموا   = Verily they didn't speak or   They will never speak.

بلى   means   نعم   =  Yes.

Generally it is not familiar in speaking Arabic, although it was used in Quran.

Hope it will help you.