If your sentence was not accepted it was not due to the use of "trousers" as mizinamo has already told you. There must have been an actual error.
We moderators cannot see your sentence therefore before you posted your comment here you should have made a REPORT.
Here's how to report.
Go to the bottom of the exercise page where you'll see REPORT Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct.
-The dictionary hints on hover are wrong or missing.
-The Greek sentence is unnatural or has an error.
-The "Correct solution" is unnatural or has an error. -My answer should be accepted.
This is the one you should click on if your translation was not accepted. After choosing the REPORT you want to make you should post on the comment page.
"Pants" are one of a small group of words that are always plural in English. The word "pant" doesn't exist in English so the singular is "a pair of pants".
See my comment here. for more information on these words.
Here is another comment about pants.
There are twelve comments on this page that say:
The word "pants" is always plural in English.
Please read the comments.
There are certain words which seem singular that are always always plural in English. Here are some others. The singular for these words requires a special expression which I've listed with each:
Pants, trousers ( shorts, leggings, jeans, flares, tights, overalls, and dungarees) For singular we use a pair of pants, a pair of trousers, etc
clothes ... For singular, we use an article of clothes
glasses .... For singular, we use a pair of glasses (also, binoculars, spectacles, and goggles.)
Scissors.... a pair of scissors
There are a few others but these are the most important.