I know we were planning to go to the park, but it has started raining.
We can't go to the park now because it's raining outside.
ーI know we were planning to go to the park, but it has started raining.
「公園に行く予定だったけど、雨が降ってきちゃったね」
to plan to go to the park で「公園に行く予定である」
to start raining で「雨が降り始める」
ーWe can't go to the park now because it's raining outside.
「外雨降ってるから、公園に行けなくなっちゃったね」
it's raining outside で「外は雨が降っている」
ご参考まで!