たらいい・といいIt would be nice if, It would be good if, Should, I hope

ないほうがいい is used to advise against doing something, similar to 'You shouldn't…' in English. たらいい and といい can also be used to give advice, but they often express the speaker's wish or hope for something to happen, similar to 'I hope' or 'It'd be nice if…'. Unlike ないほうがいい, they can be used for situations outside a person's control.

All three expressions share the base meaning of 'it would be good if (A)' and can be used to express wishes or give suggestions. たらいい・といい tend to sound stronger or more assertive than ばいい, which feels lighter or less serious in tone. While often interchangeable, in questions like どうすればいい？・どうしたらいい？ ('What should I do?'), といい is not usable.

たらどう is a gentle, open-ended suggestion, e.g. 休んだらどう？('Why don't you take a break?'). たらいい・といい can also suggest, but sounds more like advice, e.g. 休んだらいいよ ('You should take a break.'). It can also express hope, like 雨が止むといいね = 'I hope the rain stops.'.

ばよかった and たらいい・といい are similar in that both express the speaker's wish or hope. ばよかった is used when expressing regret over something that happened (or didn't happen) in the past, e.g. 行けばよかった ('I wish I had gone.') which implies 'but I didn't'. In contrast, たらいい・といい typically expresses something the speaker hopes will happen in the future, e.g. 明日晴れるといいけど ('I hope it's sunny tomorrow.').

たほうがいい is a common expression used to give advice or make suggestions, often reflecting the speaker's personal opinion on what would be better. たらいい・といい carry similar meanings, but often expresses the speaker's wish for something to happen, similar to 'I hope' or 'It'd be nice if…' Because of this, only たらいい・といい can be used with things out of a person's control.

Both といい and たらいい can express 'if you do (A), it would be good'. but differ in nuance. たら is less certain, often used for things that may or may not happen, e.g. 'It would be nice if (A).'. と is more definite and can imply hope, e.g. 'I hope (A).'. Depending on the context, といい can also suggest 'You should (A).'.

はずだ expresses confident expectation, as in 雨が降るはずだ ('It should rain.'). たらいい and といい express hope or desire without implying it will happen, e.g. 雨が降るといい ('I hope it rains.'). The difference lies in expectation vs. wish. Thus, といい is often used even when the speaker is not confident the thing will happen.