comma before "and", "but"

[user_gary]

I wonder, why, sometimes a comma precedes the conjunctions like and, but etc. but sometimes not.

Could you explain why it so?

[Tdol]

Have a look here: http://www.usingenglish.com/forum/tags/oxford-comma/
Also, if and/but are followed by an independent clause, we generally use a comma.

[belly_ttt]

How about a full stop?

[Anglika]

A full stop is exactly that - the full stop at the end of a sentence. It ends a sentence.

[belly_ttt]

Can we put But after a full sto[p?

[Tdol]

Yes, we can. However, in formal language, it is often better to use something else.