"these trousers" or "this pair of trousers" ?
"Pants" means underpants - in the UK at least...
Pants is what we say for trousers in the US. I've found the Greek course to often accept both US and UK words and usually recommend the British-isms, but I guess not here.
that would make it difficult to use "pants" as a verb :-)
Why "αυτο το παντελόνι" and not "αυτα τα παντελονια", as the english is expressing the plural?
If you're translating from English to Greek, then yes, pants can be either παντελόνι or παντελόνια. From Greek to English, παντελόνι is still pants but it refers to a single pair.