Location: Near Cambridge UK and Near Questembert, France
Posts: 22,754
Er - "can you use an existing song written by someone else for songwriting"
In what way?
Obviously you CAN use everything from melody to rhythm track to hooks to lyrics.
But in each and every case the law is pretty clear.
You would have to pay royalties to whoever ACTUALLY wrote the song.
Unless of course you remove enough of the original to make it 100% original, in which casw why mess with the original hit in the first place?
Maybe you need to describe what you plan on doing?
if you like the way things are arranged sure you can take midis and move stuff around and experiment - try changing some chords around, throwing a different melody on top, etc...
You can certainly analyse the MIDIs (in the same way you could with sheet music) to see how the thing is put together, what chord sequences are used, how verses and chorus fit together etc. You might even be able to get some arrangement hints by working out more or less what instruments are used and how they're put together.
What you can't do is use anything directly from the MIDI in any tunes you publish without running risk of being accused of plagiarism (stealing basically).
Of course if you're only producing songs for your own entertainment and not publishing them, not even on places like Soundcloud, then you can do anything you like. No-one will ever know.
I make covers of classic songs I like for my own amusement. I'll often start from a MIDI file I've found online, and either use it as a guide for the song structure or replace some tracks with live playing and use others to trigger VSTs. I'm not a drummer, so I often just send the drum tracks to EZ drummer and make minimal adjustments. I realize I'm using someone else's work at that point, but like I say it's just for my hobby, and it was lying around the internet anyway.
I also use them to figure out complicated parts that I can't make out from listening to the recording. Sometimes the person who's made the MIDI file has figured it out, sometimes not.