YOU SAID:

the rain in spain stays mostly on the plane

INTO JAPANESE

スペインの雨はほとんど飛行機にとどまります

BACK INTO ENGLISH

Most of the rain in Spain stays on the plane

INTO JAPANESE

スペインの雨のほとんどは飛行機にとどまります

BACK INTO ENGLISH

Most of the rain in Spain stays on the plane