Each mode is a scale, Guest. When you sing a song, all, or at least almost all the notes come from one scale.

The intervals between the notes allowed in the scale are different in each mode.

If you can even find a picture of a piano keyboard and name the keys, you can see the mechanism using Bonnie Shaljean's suggestion above.

Most of our western songs in a major scale have intervals between the notes that go like this (in units of "half-notes"):

2-2-1-2-2-2-1

which you can see for yourself just by playing the major scale on any string.

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