Peter wanted to speak but his words

[Bassim]

Are my sentences grammatically correct?

Peter wanted to speak but his words clogged in his throat. He stared disbelievingly at the doctor, who few moments before told him he had liver cancer, hoping he would say he was only joking, but the doctor's face remained impassive. It was incredible that he who had never drunk nor smoked could catch such a terrible disease.

 

[Charlie Bernstein]

Are my sentences grammatically correct?

Peter wanted to speak, but his words stuck in his throat. He stared disbelievingly at the doctor, who a few moments before had told him he had liver cancer, hoping the doctor would say he was only joking. But the doctor's face remained impassive. It was incredible that he who had never drunk nor smoked could catch such a terrible disease.

The grammar was all fine.

Commas should separate the parts of compound sentences, but your first sentence was short enough that you could get away with skipping it. I put one in because, strictly speaking, it's called for.

"Stuck" would be more natural than "clogged." (And we wouldn't use the phrase "clogged in" there, we'd say "clogged his throat.")

You also have more "he" pronouns than most editors would like. Your meaning was clear, but it's good to come back to earth with a real noun (like doctor) from time to time.

And watch out for run-on sentences. You're not James Joyce. (Yet!)

 

[Bassim]

Charlie Bernstein,

Is it OK to start a sentence with "But," or is it better to use a comma before and start the sentence with "but"?

 

[Roman55]

I don't think that one catches cancer. That gives the impression that it is infectious or contagious. Perhaps contract or develop would be better.

 

[Charlie Bernstein]

Charlie Bernstein,

Is it OK to start a sentence with "But," or is it better to use a comma before and start the sentence with "but"?

It's fine to start with a conjunction. It's also fine to use a comma and conjunction. When a compound sentence is too long, an easy solution is to break it into two.

There is no metric for deciding when sentence is a run-on. It's a judgement call. But (See that? Another But!) I think most writers, English teachers, and editors would agree that that was a good place for a break.

One way to think about it is to ask how closely the parts of the sentence are related. You started with Peter staring and shifted to how the doctor's face looked. So you shifted the sentence's topic. Again, there's no law against doing that, but your images might have more force if you allow them to stand up and breath. Shorter sentences can do that.