ことだ

Gives strong, direct advice: "the best thing to do is…" or "you should…". It is often paired with a condition (〜なら, 〜たければ) that states the listener's goal.

Speakers use it to give advice with conviction, typically to equals or juniors, as when a senior, teacher, or doctor tells someone what they ought to do. It commonly follows a goal clause such as 〜たいなら or 〜たければ. It is more assertive than ほうがいい but is advice rather than a direct command like なさい.

• ほうがいいN5「ほうがいい」also gives advice, but in a lighter tone with no preachy feel; it usually attaches to the た形, though the 辞書形 and ない形 are also possible. It can also express a preference within a comparison (e.g., 「青のほうがいい」), a use that 「ことだ」does not have.
• べきN3「べき」expresses what reason or social norms say one ought to do; it can discuss principles in general terms and can also modify a noun, as in 「やるべきこと」, whereas 「ことだ」mostly offers advice on the listener's specific situation. Their negative forms are 「べきではない」and 「ないことだ」respectively.
• なさいN4「なさい」is a command or an instruction in exam questions, and attaches to the ます形 with 「ます」removed; 「ことだ」is advice, attaching to the 辞書形 or ない形 and often paired with 「〜たいなら」. Even as a command, 「なさい」cannot be used toward superiors, and the two should not be interchanged.
• ものだN2The admonishing sense of 「ものだ」is grounded in social common sense, saying 'in general, this is how one should act,' so it is broader than 「ことだ」and not tied to a specific situation. 「ものだ」can also look back on the past with nostalgia or voice emotions, uses that the advice-giving 「ことだ」does not have.
• 〜ことにしているN3「ことだ」urges the listener that 'this is the best thing to do,' while 「〜ことにしている」describes a habit, principle, or fixed arrangement that you or someone else maintains by personal decision — it is not advice. Both attach to the 辞書形 and ない形, but their meanings differ and they cannot be interchanged.

ほうがいい is a softer suggestion, while べき expresses moral or logical obligation and なさい is a direct command. ものだ states what is generally expected or natural as a social norm, and 〜ことにしている describes an established habit that someone (the speaker or another person) has decided on, rather than advice.