～たほうがいい
RuleVerb ta-form + hou ga ii: gives advice, "had better V." The negative is ~nai hou ga ii.
Curiosity
How do you give advice to someone, as in "you had better rest a bit," in Japanese?
Intuition
hou ("side/option") puts "this side is better" on the scale. Positive advice uses the ta-form (the weight of completion); negative advice uses nai hou ga ii. Using the ta-form is the key point.
Visualization
taberu (eat) → tabeta (ta-form) → tabeta hou ga ii (had better eat). It passes through the ta-form into advice.
食べるdictionary form
Essence
Positive: ta-form + hou ga ii (yasunda hou ga ii); negative: nai-form + hou ga ii (tabenai hou ga ii). Using the ta-form for the positive is characteristic (stronger advice than the present). The compared option can take yori. Politely: ~ta hou ga ii desu yo.
Examples
疲れたら少し休んだほうがいいですよ。
If you are tired, you had better rest a bit.
夜は甘いものを食べないほうがいい。
You had better not eat sweets at night.
早めに予約したほうがいいです。
You had better book early.
Mini-quiz
Which fits the positive advice "you had better go to the doctor"?