只是【zhǐshì】

At the head of a second clause, 只是 adds a reservation to what came first: "it's just that", "only". The first clause stays the main point; the 只是 clause is the one small problem.

只是 or 但是

但是 sets up a real contrast and makes the second clause the point. 只是 is softer and keeps the weight on the first clause.

不过 is close to 只是 in tone.

"Only, merely"

Inside a clause, 只是 before a verb phrase or a noun means "only", "merely" – nothing more than this.

只是 or 只

只 + verb + object limits the object: only this, nothing else. 只是 limits the whole action: only doing this, nothing more serious.