So far, I have learned about three different ways to say "should" in Japanese. I'm interested in knowing the differences among ほうがいい、べき and ことだ. What are some situations that each form would be better suited in? Isn't べき more formal and stronger?
お菓子を食べすぎないほうがいい。(I've noticed that ほうがいい is sometimes translated to "it's better to do")
お菓子を食べすぎないことです。
お菓子を食べすぎるべきではない。
All translate to "You shouldn't eat candy too much" but what is the nuance in each?
ほうがいい
Is used to say that out of a number of possibilities the stated one is preferable.
お菓子を食べすぎないほうがいい
It would be best if you didn't eat too many sweets.
ことだ（です）
Is used to say that something is important.
（虫歯が嫌だったら）お菓子を食べすぎないことです。
(If you don't like getting cavities then) not overdoing the sweets is a key.
しないこと。
Is used to state rules in a stiff, harsh fashion: "One is not to..."
（ダイエットの理念として）お菓子を食べすぎないこと。
(As related to a diet book or something) You are not to eat too many sweets.
べき
Fundamentally states what the natural state of something is expected to be; other derivative meanings come from that general idea. In this case it's almost like stating a duty / obligation / rule.
お菓子を食べすぎるべきではない。
A person shall not eat too many sweets / people are not supposed to / not meant to eat too many sweets.
In order from soft to strong
～ほうがいい - should (nuance: soft suggestion, "this way is better/best)
～はず - should (nuance: supposed to, ought to, with no expectation from the listener)
～べき - should (nuance: supposed to, with expectation that the listener would follow)
～こと - should (nuance: almost like a command. can be softened with ね, can be stronger with だ / よ)
