Hello!
I have a question for you.
I found a chords progression in Am which goes Am | Em | D | C |
So now my question is: why does D pass into the Am and the whole chords progression when it involves F#? Or is this chords progression in other Key?
Thank you for your answers!
Br Daniel
Hi, I think you can consider it like the II grade of G, I mean Am Dorian, where the sixth grade is major and not minor like in the Am natural. Look at this
Am Dorian: A - B - C - D - E - F# - G
Am Natural (Eolic): A - B - C - D - E - F - G
I know that in terms of Music Harmony isnāt right, but itās G Major scale where you can find F#
However in you melody you can find F for Am, Em and C, but not in D (D-F#-A), but this is another thing more complicated 
I hope I helped you
I have edited this post as I realised afterwards that I left my answer incomplete. In western music the A minor scale has three variants:
Natural minor - A - B - C - D - E - F -G -A
Harmonic minor - A -B - C -D - E - F - G# - A
Melodic minor - A - B - C - D - E - F# -G# - A
Generally the melodic minor scale is used in a ascent of the notes and a D major works. But what we are missing here, and what I should have explained earlier, is that when working on chord progression you are not stuck to the notes/chords of that scale. You can āborrowā chords from related scales.
For example in A minor you can borrow from the A major scale. So this opens the door to playing any chord that contains the notes C#, F# and G#. D major contains D F# and A, and is the subdominant of the key of A Major. Similarly E G# B, although appears in the harmonic minor scale you can look on it as the dominant of the Major scale.
Therefore, you can use the major IV chord when you are in any minor key and any minor IV chord when you are playing in the major key.
Trust this helps.
Technically this works because of two reasons.
The first is because you are in E minor. You are just starting on the 4th degree of the scale which gives the illusion of A Ionian (without the C# and G#. This means that you can switch between major and minor for three chords A, D and E.
The second and more simplistic reason is you are creating a strong perfect cadence in D Major. Looking at it this way then allows you to create a smooth baroque or classical key change from D Major to another related key.
Alternatively you could just be boring and look at it as Am melodic minor which is kinda cool. You raise the F and G on the way up and flatten them on the way down 
Hope this gives you some understanding and also a bit of context 
To give a correct answer I need more than a group of Chords or small chord progession that belongs to anywhere. you do not mention the Key signature of the song where you find these chords.
With these in mind, I agree to see D major as a part of a dorian mode. I agree, because I suppose
A minor is the key signature. I hope this is the case here. The other scales of A minor for example: melodic or harmonic minor have the g sharp. This means you can not use E minor or C major chords. In the case of melodic minor you have on the third a C aug chord. Therefore in melodic and harmonic Minor there is no C major chord at all.
The next step is to look to the main key signature. But it is no conflict to have aeolian and sudden the D major chord comes in. To use a chord of an other scale is quiet normal in a lot of compositions. The name of this is āmodal interchangeā.
I give a chord progression of famous song called āHouse of the risingā - It goes like this:
Am - C - D - Fā¦ and so forthā¦
I give you a chord progression of famous song from the 80er " Everybreath you take"
The song as far as I play it is apparently written in G major. But the Bridge includes Eb major and F major.
How does fit to G major?
I hope I could make the things a little bit clearer.
Klaus Ferretti