>Thanks, Hilmar. I don't know, why I was so confused today. Ok, it would be quite interesting to find similar words in English, where the single and plural forms are the same. And also if the word is in plural, but used as a single.
>
>What are your examples?

Sheep
Hair
Pair [an "odd" one at that, although pairs is an accepted plural as well]
[Eye] Glasses
Pants
Scissors
Fish

Then you have phrases like:

a pair of scissors/pants/eye glasses, 2 pair of scissors/pants/eye glasses