YOU SAID:

I told my dog put the keys inside of the mattress

INTO JAPANESE

私は犬にマットレスの中に鍵を入れと言った

BACK INTO ENGLISH

I told the dog to put the key in the mattress.

INTO JAPANESE

私は犬に鍵をマットレスに入れるように言った。

BACK INTO ENGLISH

I told the dog to put the key in the mattress.