When do I use the comma?
Before I go I will return the book.
Do we have to use a comma in the above sentence?
I would, because the first three words constitute a ‘weak interruption’ to the sentence. This is perhaps clearer with a slight re-ordering of the words to read ‘I will, before I go, return the book.’ The point about a weak interruption is that it can be removed without destroying the sentence. ‘I will return the book’ doesn’t give as much information as the fuller version, but it remains a grammatical sentence.