W moim piórniku są cztery kredki.

There are four crayons in my pencil case.

Breakdown of W moim piórniku są cztery kredki.

• być: to be
• mój: my
• w: in
• piórnik: the pencil case

Questions & Answers about W moim piórniku są cztery kredki.

Why does piórnik become piórniku after w?

Because w meaning in usually requires the locative case when it refers to location.

So:

• basic form: piórnik = pencil case
• after w: w piórniku = in the pencil case

This is a very common pattern in Polish:

• w domu = in the house
• w sklepie = in the shop
• w piórniku = in the pencil case

So the ending changes because the noun is in the locative singular.

Why is it moim piórniku, not mój piórnik?

For the same reason: the adjective or possessive word has to match the noun’s case, number, and gender.

The dictionary form is:

• mój piórnik = my pencil case

But after w, the noun goes into the locative, and mój changes too:

• w moim piórniku = in my pencil case

So both words change together:

• mój → moim
• piórnik → piórniku

This kind of agreement is very important in Polish.