It isn’t always easy to understand or to figure out how modes are built. But let’s first precise some basic notions of the scales and modes.

A scale consists of seven notes and so is the number of modes. The first and last note of the scale gives the tonality, whereas the intervals give the mode.

Now, let’s take a close look at the two octaves C major scale (The Root Scale) in the diagram below.

This is from where you can infer the rest of the other modes. See now the other modes. They have exactly the same interval as the C scale. The only difference are their first notes. (The first note of a scale determines the tonality and its interval determines the mode) If you take the second scale, which is in D Dorian mode. It has the same interval as the C scale. The only difference is its first note (D). You can see it as it has been shifted up of one note. For instance, from G to G you'll have a G scale in Mixolydian mode and so on.

So, what is crucial here to understand is the result of that shift. With C Ionian, we have the first Third that is Major. But now with D Dorian the first Third is minor. This is the purpose of the modes. It is to give to each mode its own coloration.  

Now, all this wouldn’t be completed without an application point.

If you analyze all the modes from the C root, you will easy see that the A Aeolian mode is minor and is also the “complement” of C major.

That’s particularly interesting when improvising. You’ll have then a different and yet complementary mode at your disposal. This applies in all keys. For instance, in G major, you have E Aeolian (minor).

A good reason to practice scales in both major and minor modes.

The construction of modes