Fetching details from the dictionary...

ordinary; common; usual
free of charge
unaffected; as is; safe
only; merely; just; simply
but; however; nevertheless
ただの水でいいですよ。
Just plain water is fine.
彼はただ見ているだけだった。
He was just watching.
ただ事じゃないと直感で感じた。
I felt instinctively that something was wrong.