prefixes - How to speak about "one" thing in Japanese?

I know the title does not make much sense, sorry about that.

In short, what I mean is, how can I mention about one certain thing (not one thing in amount) in Japanese?

For example "one morning" in the following sentence:

As Gregor Samsa awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect.

I come accross some usages like "あるX". For instance:

• One morning -> ある朝


• One day -> ある日

Is this ある may be what am I looking for?

Sorry if I am not clear. This is all I could express myself.
Thanks.

Answer

As Gregor Samsa awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect.

One morning -> ある朝

Is this ある may be what am I looking for?

I can confirm it is the one, as a native speaker.

The kanji version is 或る, but more often it appears in ひらがな: ある人が…, ある時, あるところにおじいさんとおばあさんが, etc.

We also sometimes say とある: とある日曜の午後 (with rather a feel of popular literature).

某｛ぼう｝ is yet another possibility, in the same manner: ex. 某日｛ぼうじつ｝, 某会社社長{ぼうかいしゃしゃちょう}, 某ファーストフード店｛てん｝, etc.