Perhaps I am just remembering previous lessons incorrectly, but I thought Chłopcy was a singule boy and Chłopcem was multiple boys? As a native English speaker, I find all the different changes in vet's and nouns very difficult.
"Chłopcem" is the Instrumental form of "chłopiec", used when saying "[...] is a boy", and for some other things too. Meanwhile "chłopiec" (boy) and "chłopcy" (boys) are Nominative forms, that is they are used when the noun is a subject.
You have to know your basic language. So start with English, if there is not a Iranian/Polish version on Deo.
'I am from Iran, help me. '
It will not except wrong spelling.
English is also my second language.