HOMEWORK CORRECTION: 

A.

I think it's good (no punctuation/no capitalization if followed by the word 'but')​but I can always make mistakes, so I have to be careful. ✔

=> Corrected: The sentence is corrected grammatically.​

B.

Hi Doldol, are you free now? ✔​​

Yes. ✔​​

How about going to see a musical?​ (punctuation)✔​​​​