In English we have a select category of nouns which always take the plural form. Often, we try to quantify these nouns by adding further qualifying words to a sentence, but they always retain their plural form. We can say 'I like your trousers', just as we can say 'I like your pair of trousers', and, if I was complimenting a crowd of people on their fantastic trousers, I would still use precisely the same sentence ('I like your trousers').
There are words with exactly the same character in Polish, and they seem to emanate from the same logic, so can appear to be pretty similar across the board. Spodnie in Polish means 'trousers' in the Plural, meaning the same example sentence - Lubię twoje spodnie (I like your trousers) - would also contain the plural form of the word by default. Words like this are important because in Polish you will need to make alterations to the grammatical structure of any existential verbs that you use in conjunction with these ever-pluralised nouns. It's worth noting that English does precisely the same: 'These are my glasses' and 'This is my glass'.
in fact, forming the plural of existential verbs in the present tense in Polish is virtually identical to how it's done in English. All that's needed is to exchange the polish singular jest (is), with the Polish plural są: 'To są moje okulary' (those are my glasses), and 'to jest moja szklanka' ('that is my glass').
The only thing that can get tricky in the present tense singular is that the interrogative forms of questions addressing plural subjects, or ever-plural nouns, can remain in their original singular form. So, it is acceptable of me to ask 'co to jest?' (what is that?) of plural objects, but not correct of you to answer using the singular jest. Same goes for the personal interrogative kto (who); if the subject is plural then my question 'kto to jest?' (who is that?) still demands the plural indicator 'są'.
The plural form of existential verbs in the past tense is a little harder, but this is only to do with the rules surrounding the formation of past tense verbs per se, and not to do with any plural/singular confusion. In this instance you will need to match the ending of the past tense verb form with the correct gender of the object noun.
So, if I wanted to say 'those were my brothers', I'd use the past tense plural masculine (as brat is a masculine noun) form of the verb 'to be', giving To byli moi bracia (Those were my brothers). Just bear in mind there are different past tense plural forms for feminine, masculine, masculine animate and neuter nouns, so there's a few verb ending changes to get used to here.
Finally, it's worth mentioning some of the nouns which you will be required to use plural existential verb forms for, because, although I began by saying they seemed to follow similar rules to the English ever-plural nouns -where trousers have two trouser and glasses have two lenses, so on and so forth - there are some which may seem a little odd.
For example, lody (ice-creams), kleszcze (clippers), drzwi (doors) widły (fork), and tapety (wallpapers), all, with varying degrees of appropriateness, fall into this category.