Putting "comma" before "but"

Question

Suppose I wrote the following:

She did well in the test but Tom didn't.

Why there is no comma before "but" whereas we put it in the following example:

I saw him before war, but I haven't seen him since that time.

Answer 1

There actually are many rules governing comma usage in English. The reason there's no comma in your first example is writers choice however. In your first example the sentence is so short that you can avoid using the comma before the conjunction but. In the second sentence because it is longer and more involved, the writer must put the comma.