This is the most simple addition reaction, mostly because this is the example that we use to teach the mechanism in the first place. You should already be familiar with this reaction.

General Reaction:

Note: The squiggly line on the product just means “indeterminate stereochemistry”. We aren’t sure where that –H will add, so we’ll just draw it on a squiggly line. 

Example: Provide the mechanism of the following addition reaction.