Learn Polish: Pamiętajcie, że jutro jest ważne spotkanie. - Remember that tomorrow is an important meeting.

It is addressed to a group of people, like saying "Remember, you guys!". The dictionary form is pamiętać. To form the plural command, you start with the singular command pamiętaj (remember!) and add the plural ending -cie.

If I want to tell just one person to remember, how does the sentence change?

You just remove the -cie ending from the verb. The sentence becomes: Pamiętaj, że jutro jest ważne spotkanie.

Why is there a comma before że?

In Polish grammar, you must always put a comma before że (that) when it connects two phrases. This is a strict rule, unlike in English where the comma is usually left out.

Why does the adjective ważne end in -e?

It describes the noun spotkanie (meeting). Because spotkanie ends in -e, it is a neuter noun. The adjective must match this gender, so the base word ważny changes to its neuter form ważne. Both words are in the Nominative case here, acting as the subject of the second half of the sentence.