唯一つ

ただひとつ【唯一つ】

●ただ一つの single; sole; only; but one

・ただ一つの短所 only one weakness [fault, defect].

●方法はもうただ一つだ. There is but one way left.

・ただ一つ言っておきたいことがある. There's (only) one thing that I want to tell you.

・ただ一つ残念なのは彼女に再会できなかったことだ. My only [one] regret is that I couldn't see her again.