Do you need a comma before so when it joins two sentences together? Because English is rarely straightforward, it depends.

Here’s a trick:

Try saying “so that” instead of so. If it works, you don’t need a comma.

Example:

Mary went downtown so she could renew her driver’s license.

You could say, “Mary went downtown so that she could renew her driver’s license.” Therefore, no comma required.

What about this one?

Example: 

Mary had to go downtown to renew her driver’s license, so she decided to make an afternoon of it and visit the art museum.

In this case, “so that she decided” wouldn’t make sense. Use a comma!