YOU SAID:

Thunder only happens when it's raining

INTO JAPANESE

雷は起こるだけ雨が降っているとき

BACK INTO ENGLISH

Is raining Thunder only happens when

INTO JAPANESE

雷は雨だけが起こるとき

BACK INTO ENGLISH

When only the rain Thunder

INTO JAPANESE

ときだけ雨雷します。

BACK INTO ENGLISH

Only when the rain Thunder.

INTO JAPANESE

雨雷の時のみ。

BACK INTO ENGLISH

Only when the rain Thunder.