#1
From my understanding, If you're playing, in example, in the key of G, then you can play the G mixolydian mode and still be in the key of G (If I am correct). But the G mixolydian has an F instead of an F# so wouldn't that place the whole mode out of the key of G?

Example in easiest terms possible please

thanks
#2
mixolydian is the 5th mode and the 5th of G major is C, so you play the notes of C major which has a F instead of an F#
#3
No, you will have only played one note that's out of key but that doesn't mean it's wrong.

What sounds dissonant ("bad") more often depends on the chord/notes that you're playing over so if you're playing over a G5 then it'll sound fine but if you're playing over something like a Gmaj7 then it'll sound a bit off.

Ultimately the notes that you play in theory don't matter because music theory isn't really a set of rules: it's guidelines for what should sound ok and a set of rules for describing what you play after the fact.
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#4
When you say "key of G", you are referring to G major, with an F#. Playing in a different mode, like Mixolydian, it is no longer G Major; it is G Mixolydian, with an F natural. It still has G as the root, it's just not a "G major".
#5
No, you will have only played one note that's out of key but that doesn't mean it's wrong.


Correct. This was one of my misconceptions when I was first learning modes. I thought that a D Dorian scale (DEFGABC) was in C major and thus could only be used a substitute for C major. Dont think of it as being in C major, think of it as being in D minor. Same applies to the G mixolydian scale.