I was walking in the 樹海 (jukai), the "Sea of Trees", also known as "the suicide forest" with Rei (then girlfriend, now wife) on one of our first dates.
I thought that I saw some mysterious figure (a dead body?!), so I stopped and looked a little more closely.
何？ (nani? // What [is it]?), Rei asked.
I realized it was just a rock, so I said:
ishi dake da
Literally, "Rock only is."
In my English brain I was saying, "It's just a rock."
0.3 milliseconds after I said that, she corrected me by saying:
tada no ishi da
It's just a rock.
This was the first time that I learned the difference between ただ (tada) and だけ (dake).
Nothing helps you remember something better than making an embarrassing mistake in front of a girl you on your third or fourth date, right in the depths of the suicide forest, no less.
The Difference Between ただ (tada) だけ (dake)
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.
だけ (dake) is for numbers or amounts
I don't really know of any way other than to say this, but the Japanese だけ (dake) is the same as the words "just" or "only" in English when they express the (limited) amount of something.
ただ (tada) is for "nothing special."
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).
It might help to think of ただ as saying "nothing (but)," because it can also mean "free; for nothing:"
ただ (tada) and だけ (dake) can be used together?!
This can be done when saying that there is "only" (だけ) a certain amount of something, and that amount is "nothing special" (ただ).
Example sentences, please
You can find them over at NativShark.