YOU SAID:

i need my mouth to be full

INTO JAPANESE

口がいっぱいになる必要がある

BACK INTO ENGLISH

mouth needs to be full

INTO JAPANESE

口がいっぱいである必要がある

BACK INTO ENGLISH

mouth must be full

INTO JAPANESE

口がいっぱいでなければなりません

BACK INTO ENGLISH

mouth must be full