spodnie = a pair of trousers
Trousers are a piece of clothing that covers your body from the waist downwards, and covers each leg separately.
Trousers is a plural noun. You use a plural form of a verb with it.
Don't talk about 'a trousers'. You say some trousers or a pair of trousers.
It's better to stick to strict translations in the beginning, and only later start to figure out the exact semantic boundaries between words.
I'll copy a comment I wrote elsewhere:
Kapelusz usually has an all-around rim, czapka doesn't. Kapelusz is usually rigid, czapka usually is either flexible or at least fluffy.
Going by the list here: https://en.wikipedia.org/wiki/Hat#Styles
Kapelusz can refer to bicorne, bowler, derby, sombrero, conical Asian hat, fedora, montera, panama, pillbox hat, top hat, tricorne
Czapka can refer to ascot cap, balmoral bonnet, baseball cap, beanie, bearskin, chullo, cricket cap, coonskin cap, Phrygian cap, rastacap, Santa hat, toque, tuque, ushanka
Czapka may refer, although it's better to you a more precise word, to a beret. (Beret in Polish is beret.)
Neither can refer to custodian helmet, fez, keffiyah, hard hat, kippah, kufi, mitre, pith helmet, turban, zucchetto.
I have to clue how to classify a deerstalker, but I think it's czapka.
Need to save this list of headware types, I am not sure I know a half of them (though about some of them I guesses what it is) Still I wonder about spodnie, in Russian and Ukrainian the alike words describe mostly underpants or generally underware, does it mean both underpants and usual pants in Polish? Does it mean just clothes we put on our legs? I mean there's a type of retro underware, covering the whoke body, legs, arms , shoulders, or the type of sleeveless trunk (t-shirt). So you can't describe those as spodnie, can you?