What you need to do when you take a look at the chord chains you see and say: "This is a simple song C major," and then you say: "Oh no, E7 doesn't belong to the scale because it has no G#."
We want to help you figure out the scales when you're trying to play diatonic songs with a couple of chords that don't come from the scale.
So here's a simple hack for that
First of all, every time you have a chord that doesn't exist in the scale, do the following:
Take the chord tones, for example, E7, and the chord tones are E, G#, B, and D and try to fill the spaces between them with notes from the scale; for example, between E and G#, you could have the C major scale note of F that's the beginning of A scale
Next, you may have between G# and B the scale notes from C major A, and between the notes B and D, you have the scale note C, and the resulting thing is a full scale which is sometimes called Mixolydian b9 b13, and some people recognize that it's a mode of A harmonic minor.
So let us recap the concept. When you sense that you are in a specific scale but see a chord that doesn't fit, take its chord tones, one-three-five-seven, and fill in the gaps with notes from the scale. That happens because once you have a foreign chord, you want to make it as least dissonant as it needs to be because it stepped out. So you mix the notes in the scale and the notes you have to accommodate.
Here is a second example.
The 3rd chord is A7 which is not a diatonic chord and does not appear in the C major scale. You can use "All of me" again.
Step one: take the chord tones of A7, A, C#, E, and G and fill in the gaps with notes from the C major scale. Between A and C#, you can use B. Between D and F, you can use E, and between E and G, you can use F to get the scale.
That's a good solution. A Pro tip is to think of it when you have a non-diatonic chord. Just fix the notes you must include for your chord with notes you already have. We hope this helped you. Watch more helpful guitar lessons on our YouTube channel and subscribe to our blog.