Quoting this article on Russian plurals:
If the number is 1, or the number ends in the word “один” (example: 1, 21, 61) (but not 11), then you should use the case (singular) that is suitable for the position in the sentence. The number one declines like an adjective.
What exactly does that mean? Are numbers ending with 1 used in singular ?
Edit: I would appreciate it if someone could provide some examples of those numbers in proper sentences.