When should you swing, and when should you play it straight, that is the question. Though experience and using your ear are part of it. If you are playing with written music, it is indicated at the top left of the music, just under the title.

Here are a couple of different ways that you will see this indicated.

The words swing, swing feel, swing 8ths, shuffle, shuffle feel, triplet feel... all indicate that you should be swinging.

The other important element is this

This means that 8th notes are played like the first and last part of a triplet.

back the beginning of the swing strum story

What the heck is a triplet, and what does that have to do with swinging strums