Some notes on the use of personal pronouns:

On and ona, meaning he and she respectively, can refer to all kinds of nouns, not only people. It depends on the gender of the noun: masculine nouns are referred to as on, and feminine nouns as ona. Ono refers to neuter nouns. It’s the same principle as in, for example, German: every noun has a gender (even though there’s nothing inherent in a potato that makes it masculine) and has to be referred to with the appropriate personal pronun.

Oni (they) refers to a group of people including at least one male. If the group consists of a dozen women and one man, you’ll still have to use oni to talk about them. Which sucks from the feminist point of view.

One (they) refers either to a group of people consisting exclusively of females, or to a group of inanimate beings. Which sucks even more when you realize women are put in the same category as things and stuff.

What do these words have in common? For one thing, they’re easy to recognize because of their –um ending. Even though the list isn’t comprehensive, you shouldn’t have problems recognizing new words of this type when you see them.

Secondly, all of them follow the same easy pattern for building the nominative plural form: the ending changes from –um to –a. Here’s how the other case forms are created, on the example of the word liceum – high school.

Nom. licea
Gen. liceów
Dat. liceom
Acc. licea
Ins. liceami
Loc. liceach
Voc. licea!

akwarium – aquarium
W klasie są dwa duże akwaria.
In the classroom there are two big aquaria.
Przyglądam się tym akwariom.
I watch these aquaria.

centrum – centre
Centra dużych miast są często niebezpieczne.
Centres of big cities are often dangerous.
Zwiedzamy centra europejskich stolic.
We visit the centres of European capitals.

gimnazjum – junior high school (grades 7-9)
W tym mieście są trzy gimnazja.
There are three junior highs in this city.
Ci chłopcy chodzą do różnych gimnazjów.
These boys go to different junior highs.

hospicjum – hospice
Hospicja opiekują się nieuleczalnie chorymi.
Hospices take care of the terminally ill.
Wiele hospicjów ma problemy finansowe.
Many hospices have financial problems.

kolokwium – test or exam in a university course
W tym tygodniu mamy dwa kolokwia.
We have two tests this week.
Ci studenci rozmawiają o kolokwiach i egzaminach.
These students are talking about tests and exams.

liceum – high school (grades 10-12)
Moje dzieci chodzą do dobrych liceów.
My children go to good high schools.
Przestępczość w liceach jest niepokojąca.
Violence in high schools is worrying.

muzeum – museum
Piszę artykuł o nowych muzeach w moim mieście.
I’m writing an article about new museums in my city.
Nie lubię muzeów.
I don’t like museums.

technikum – vocational high school
Te technika kształcą przyszłych elektryków.
These vocational schools educate future electricians.
Uczniowie którzy nie chcą uczyć się w liceach idą do techników.
The students who don’t want to study in high schools go to vocational schools.