直す, 修正, 訂正, 添削

Hi everyone.

I'm currently confused about when to use 直す, 修正, 訂正 and 添削 (of course with their respective 'suru' included)

Are they interchangeable at all?

直す is the most general. It can mean to fix (errors), to fix (physically repair), to return to a previous state, and as a compound verb, to redo an action (見直す、考え直す etc). 治す (same reading) is used for cure/heal.
修正 is for corrections or improvements, amendment, revision, etc. Not limited to text. i.e. 写真を修正する (with Photoshop)
訂正 is specifically fixing errors, normally limited to text.
添削 is similarly for text but tends to refer to when a person fixes/edits work that somebody else wrote - i.e. teacher making corrections to a student's work.

Thank you very much! So if that's the case, if I use 直す most of the time, it'll be a safe choice right?

Not always but mostly yes.