I don't think the confusion is your basic understanding of meters and time signatures, but probably how you are counting aloud with syllables.
In simple time signatures - those that subdivide the beat by 2 - it's the note value that gets the beat. Ex...
4 (four beats per measure)
4 (quarter note gets the beat)
In compound time - those that subdivide the beat by 3 - it's the value of the subdivisions. Ex...
6 (divide this by three, 6/3 = 2, there are two beats per measure)
8 (each of the 2 beat is subdivided by three eighth notes)
Time signatures are not fractions.
Time signatures are not really math.
Time signatures are short hand signs for meters. You could replace them with any other sign. Case in point: the
C for common time or
After learning how to read a time signature, it's probably more practical to just recognize the commonest by memory:
Additive time signatures like
5/4, etc. are sort of "advanced" but shouldn't be too hard to deal with provided you understand simple and compound time signature.
...it will say what kind of note gets the beat, but why do I need to know that if it won't change what I'll play? Can someone show me the difference if I play something in 5/4 or 5/8...I thought that since it was a quaver note in the bottom I would count 1 and 2 and 3 and 4...; if it was 16th note in the bottom I would count 1 e and a 2 e and a 3 e and a 4...
Technically, the person at Reddit was right to say your counting aloud is wrong. If the 16th note was on the bottom, like
4/16, then the beat count should still be "1,2,3,4" on each sixteenth note.
I think the think to keep in mind is: beats, subdivisions, beats per measure.
When using the rhythm syllables, like "1 e & a", be careful of what is a beat and what is a subdivision. There are different systems of syllables, but I think it doesn't matter much what you say as long as you keep to numbers on the beats, which is what the person a Reddit must have taken issue with in your example. Here is how I think you would count aloud for some examples...
Notice that with
5/4 the beats are subdivided by 2 so it a kind of simple meter, but in
5/8 you get beamed groups of 3 eighth notes subdividing a beat, which is like compound meters, mixed with groups of 2 eighth note subdivisions. In odd or additive meters like these the bottom number can signal the difference between simple to compound feel.
4/16 looks strange... because it is, but if you wrote it, that is how it would be counted. It's essentially common time, but you change which note values are assigned to a full beat. There probably isn't a practical reason to do this.