YOU SAID:

i have a pencil case with pens and pencils inside

INTO JAPANESE

私はペンと鉛筆が中に入った鉛筆ケースを持っています

BACK INTO ENGLISH

I have a pencil case with a pen and pencil inside.

INTO JAPANESE

中にペンと鉛筆が入ったペンケースがあります。

BACK INTO ENGLISH

There is a pen case with a pen and a pencil inside.

INTO JAPANESE

中にペンと鉛筆が付いたペンケースがあります。

BACK INTO ENGLISH

There is a pen case with a pen and a pencil inside.