PUNCTUATION RULE - COMMAS In sentences where two independent clauses are joined by connectors/conjunctions such as 'and, or, but, etc.', put a comma at the end of the first clause (before the connector/conjunction. 1 I’m sorry I haven’t answered earlier, but I was really busy working. PUNCTUATION RULE - SEMICOLONS A semicolon can replace a period if the writer wishes to narrow the gap between two closely linked sentences. The pause is longer than a comma and shorter than a period. 2 Sometimes I quarrel with my friends too; usually it IS caused by A lack of understanding. Here's a great website for punctuation rules: http://www.grammarbook.com/

Both sentences are fine as you wrote them. A comma should not be used unless there is a natural pause. For example, I would speak your first sentence without a pause. But, if you wrote the sentence with a comma before and after the "but" the sentence has a different meaning, putting more emphasis on the second part.

I would probably write: 1. I'm sorry that I didn't answer earlier, but I was really busy working. 2. Sometimes, I quarrel/argue with my friends; usually, it's caused by a misunderstanding.

1.I’m sorry I didn't answer earlier but, I was really busy working. 2. Sometimes I quarrel with my friends too; usually it is caused by a lack of understanding.