Using a Comma: Nonessential Clauses
We use commas:
1. With clauses that are not sentences.
- After three months of hard work, he managed to collect enough money.
Shortly before the match, both team expressed optimism.
2. When we want to express parenthetical thoughts/give additional information without which our sentece still makes sense.
- That man over there, who you took to be the boss, is my brother.