The basics of commas

Commas are tricky, and there is a lot to learn. <--- The comma in this sentence is here because this is a compound sentence.

However, they are predictable if you know the rules. <-- The comma in this sentence is here because of the introductory clause.

Learning all the rules will take some time, so there are a number of videos in this unit. The most basic rules are summarized below, and you can go on to the next pages in the unit to understand these rules in greater detail.