Learn Polish: Wczoraj rozmawiałem z młodymi turystami. - Yesterday I talked with young tourists.

We use the instrumental case here because of the preposition z (meaning "with"). Whenever you talk about doing an action together with someone or something, z forces the noun and its adjectives to take the instrumental case.

How is the instrumental plural formed for the noun turystami?

The dictionary form is turysta (tourist). To make it plural in the instrumental case, we drop the last vowel and add the ending -ami. The great news about the instrumental plural for nouns is that it's extremely regular: almost all nouns take the -ami ending, regardless of their gender!

Wait, turysta ends in "-a", which usually means it's feminine. Does it follow masculine or feminine rules?

Turysta is a masculine personal noun because it refers to a male person (or a mixed group in the plural), even though it has the feminine-looking -a ending. In the singular, it sometimes borrows feminine case endings, but in the instrumental plural, it doesn't matter! Because the instrumental plural is so regular, it just takes the standard -ami ending to become turystami.

Why does the adjective młodymi end in -ymi?