事(こと)だShould, Ought

たほうがいい is a common expression used top give advice or make suggestions, often reflecting the speaker's personal opinion on what would be better. ことだ may appear similar but is more formal, carrying a stronger nuance of expectation, implying 'this is how things are (or should be,' rather than a personal suggestion.

ことだ suggests what one should do as advice or a general guideline, e.g. 無理をしないことだ ('You should take it easy.') べきではない expresses the opposite, indicating what one should not do based on moral or social expectations, e.g. 人を見た目で判断すべきではない ('You shouldn't judge people by appearances.')

ことはない downplays the need for action and is often used to reassure, e.g. 焦ることはない ('There's no need to rush.') ことだ gives firm advice and carries an authoritative tone, e.g. 焦らずに進めることだ ('You should proceed calmly.') ことはない can also mean that something never occurs, which is a usage unrelated to ことだ, e.g. 彼と会うことはない ('I never see him.')

Both carry an authoritative tone, as if stating a rule, but have opposite meanings. ことだ gives strong advice or recommendations, e.g. 約束は守ることだ ('You should keep your promises.') てはならない expresses strict prohibition, e.g. 約束を破ってはならない ('You must not break your promises.')

ないほうがいい is used to suggest not doing something, similar to 'You shouldn't…' in English. It's a common expression that often reflects the speaker's personal or subjective opinion. In contrast, ことだ expresses the opposite idea, meaning 'should' or 'ought to,' and is more formal. It carries the nuance of stating a rule or general expectation rather than personal advice.

The imperative or command form (命令形) gives direct, blunt commands, e.g. 止まれ ('Stop!'). It can sound harsh unless softened with particles like よ, e.g. 頑張れよ ('Try harder, okay?'). In contrast, ことだ offers soft advice, e.g. 早起きすることだ ('You should wake up early.'), and is often used when giving recommendations or suggestions.

Both can be used to give advice or suggestions, but the tone differs. ことだ sounds more formal or authoritative, as if stating a rule or guideline. In contrast, ばいい is more casual and conversational, often used among friends. Additionally, ばいい can be used in contrary-to-fact suggestions, such as 行けばいいのに ('You should go but you won't'). ことだ cannot be used this way.

Both express what should be done, but with different nuance. ことだ is used to give firm advice, e.g. 諦めずに続けることだ ('You should keep going without giving up.') ものだ expresses general expectations or norms, e.g. 仕事は地道に頑張り続けるものだ ('One is expected to keep working steadily.')

Both express obligation, but differ in tone. ことだ is used to offer advice or general suggestions, e.g. 無理をしないことだ ('You should take it easy.') べき is stronger, implying moral or logical necessity, e.g. 約束は守るべきだ ('You should keep your promises.')

Both express reasoning in a broad sense, but are used differently. わけだ explains why something is the case, e.g. 暖房が壊れているのか。だから寒いわけだ ('So the heater's broken. That explains why it's cold.') ことだ gives strong advice or instruction, e.g. 寒いなら暖房をつけることだ ('If it's cold, you should turn on the heater.')

ことだ is used for light advice ('should' or 'ought to'), e.g. 毎日早起きすることだ ('You should get up early every day.'). なければならない expresses stronger obligation, like 'must' or 'have to', often implying consequences. ７時に起きなければならない ('You must get up at 7.').