YOU SAID:

My pajama pants caught fire.

INTO JAPANESE

私のパジャマのズボンが燃えた。

BACK INTO ENGLISH

My pajamas pants burned.

INTO JAPANESE

私のパジャマズボンは燃えました。

BACK INTO ENGLISH

My pajamas trousers were on fire.

INTO JAPANESE

私のパジャマズボンは燃えていました。

BACK INTO ENGLISH

My pajamas trousers were on fire.