write + (past) + it'd be good if you did
kaita raii
it'd be good if you did; I hope — advice for this situation, or a wish about something no one controls
meaning: it'd be good if you write
分からなかったら、先生に聞いたらいいよ。
wakaranakattara, sensei ni kiita raii yo.
If you don't get it, it'd be good if you ask the teacher. casual
頭が痛いなら、この薬を飲むといいよ。
atama ga itai nara, kono kusuri o nomu to ii yo.
If you have a headache, it'd be good to take this medicine. casual