I didn't understand this until I studied music.

Can just about anybody sing a song? Yes.

If you can sing, can you read music? Probably not, unless you have had formal training. I mean, you might be able to look at the staff and see the notes going up and say "Oh, it is getting higher here," but can you look at a piece of music you've never seen before and just sing it straight off the page? Most people can't.

OK, but let's say you can read music. Does that mean you can easily write it? That is a whole different skill that requires you to really know a lot about how music is represented in print -- what is a treble clef and a bass clef? What are the little flags on notes? What is a key signature, and what does it mean?

Speaking, reading, and writing work the same way. We all grow up learning to speak, but reading and writing are higher order skills.