Tim cannot speak English better than John This sentence tells us that that Tim's English is either worse than John's or it is as good as John's. So what do you think of a) and b)?
a) John speaks English better than Tim
b) Tim speaks english as well as John
I wonder which sentence has the nearest meaning to the original one. My teacher advised us not to use the comparision "than" in negative sentence. What your teacher said is not is not relevant to your decision here. So I pressume a is the best choice. What about you?
Put a period at the end of each sentence.