The third rule of using a comma is when combining two sentences into one using for, and, nor, but, or, yet, or so. For example: Some countries have lowered their taxes, yet the UK has increased them by 5%. However, we do not use a comma when we are using the same subject. For example: The UK has high taxes yet the average income is still low. In this sentence, the UK is the subject for both parts of the sentence; therefore, we don’t use a comma.