When it is hot and muggy, my basement smells really bad. How do I prevent/fix this?

A dehumidifer does wonders and it will also help prevent molds from growning in your basement, be sure to by one that is large enough to handle the square footage of your basement, we have one in our partial basement and it need emptied every day thru the hot humid summer months.

Get a dehumidifier turn it on high for a few days and then set it back to you feel comfortable.

A dehumidifier would be a good start.

Yes. I have one in my basement and it helps dramatically against that musty smell.

add ventilation