If you're like me and you have a bad habit of confusing these two words, then it probably means that, at least at some level, your brain is still forming thoughts in English when you speak Japanese (Like 99.99% of all students of Japanese, yeah?).To a native Japanese speaker, though, the words ただ and だけ are fundamentally different.
When I mentioned the rock in the suicide forest, it was obvious that there was only one. The "just; only" that I wanted to express what that it was "nothing special," that it was not something that's a big deal, like a body. It's "just a rock." ただの石だ (tada no ishi da).