I want to wish you a Merry Christmas

My question is: How do I know to substitute Bb for Bbmaj7 and C for C7 other than it sounds right ? Like how do I know if I my substitution should be maj 7 or dominant 7 ?

The reason that they sounded 'right' is because you chose chords that contain no notes that are foreign to the key.
Bb maj 7 contains Bb D F A - all of those notes belong to the key of F
Bb7 contains Bb D F Ab - The note Ab is foreign to the key.

C7 = C E G Bb - all notes belong to the key
C maj7 = C E G B - The note B is foreign to the key

In all major keys, the chord IV 7 is a major 7 and V7 is a dom 7

Of course, 'right' doesn't necessarily mean best or most interesting - just that they fit diatonically with the key and style of that song. In a 12 bar blues song in F though, Bb 7 would be more 'right' than Bb maj7.

Ok so another question apart from that one.

RIght so I know the "easy" keys and how many sharps they have, I know C 0, G 1, D 2, A 3, E 4, B 5, F# 6, and then well C# 7 ( the number is the number of sharps ) and I know this because of the dominants, G is the dominant of C, D is the dominant of G and so on.

Now, I know that F has one flat, how do I learn the rest of the flats like in the other keys ? And am I supposed to learn how many flats there are in the key of G if I were to write it in flats ?

As you learned the order of 'sharp' keys by matching their dominants, you can learn the flat keys by matching their subdominants
The SD of C is F, the SD of F is Bb, etc.

Or just learn them as

1 flat = F
2 = Bb
3 = Eb
4 = Ab
5 = Db
6 = Gb
7 = Cb

I'm sure you could memorise that list in less time than it has taken me to type it. :D

Google Circle of Fifths if you want some kind of understanding of how it all works. It is kind of cool actually.

Ohhhh ok so I just learn all the subdominants ( 4th note in scale )

Ahh I have seen the Circle of Fifth thousands of time, I'm not a diagram kind of guy, I understand words better :P, thanks though :).

oh, should i learn the 'sharp' keys in flats too ?

oh, should i learn the 'sharp' keys in flats too ?

No - (I'm not even sure what that means :? )

Like how to spell out G major in flats, I know that would be a complete waste of time but should I learn it, will anybody ask me about that someday ?

Can I even spell out G major in flats ?

Bah I don't think so, lol nvm I guess.

Like how to spell out G major in flats, I know that would be a complete waste of time but should I learn it, will anybody ask me about that someday ?

Can I even spell out G major in flats ?

Bah I don't think so, lol nvm I guess.

All keys have enharmonic equivilants -- that is they have the same tones but use different note names. The key of C could be also written as the key of B# -- but doing so is foolishly complex and of no real value. The notes would be: B#, C##, D##, E#, F##, G##, and A##. Likewise it could also be the key of Dbb with the notes Dbb, Ebb, Fb, Gbb, Abb, Bbb, and Cb.

However, there are a few common keys that do have multiple names which come up now and again: Db and C#; Gb and F#; and Cb with B. None of those pairs are particularly common keys for music to be written in directly, but you'll see them from time to time. Unlike the example of Dbb and B# above - which are needless complex - these paired names simply represent two equally easy ways of notating the keys.

Thank you alot, yeah I knew about there being an enharmonic equivalent but I didn't know if it was really worthwhile, but since its foolishly complex then I guess I can ignore it :D, thanks!