This isn't about the particvle "ga" (or atleast, the subject particle "ga").
I was just wondering, sometimes I'll see "ga" in the middle of a sentence, and I was wondering what it was... I don't really understand... like. I can't really give a sample sentence, but I could tell you what somethings look like:
They (most likely) mean "but". Or some kind of doubt/modesty. (Or to show that you let the listener decide; I've seen an example of it in Genki (book 1 I think it was) where a travel agency lady says something like "We have this and these tickets, が。。。"
That 'ga' can also just indicate that something is coming after the sentence. It doesn't necessarily mean 'but'. A good example of this is saying who you are on the phone 'Bucko desu ga...Sumisu-san ha irasshaimasu ka?'
explaining the difference between the two particles would require describing all of their uses. They aren't interchangeable, just go look at grammar guides that describe the functions of these particles.