Had better: nên … thì hơn

You know nên (should) before a verb. Add thì hơn at the end, and it becomes "had better", "it would be better to": Trời sắp mưa, mình nên về thì hơn. It's about to rain; we'd better go home. Thì hơn means "that would be better", so the sentence weighs one choice against another and picks one.

It's a soft, friendly way to advise someone, or to decide for yourself: Em nên đặt vé trước thì hơn. You'd better book in advance. With không nên it's "had better not", and có lẽ (maybe) in front makes it gentler still: Có lẽ anh không nên nói với chị ấy thì hơn.

Thì hơn also works without nên: Chuyện này để mai tính thì hơn. Better to deal with this tomorrow. Đi taxi thì hơn. A taxi would be better.

Compare thà … còn hơn, which picks the lesser of two bad things and sounds much stronger: Em thà đi bộ còn hơn. I'd rather walk.