certainly

Compared meaning: without doubt, of course (adverb).

比较义项：确定无疑地；当然（副词）。

• certainly 本词

  Certainly is the most neutral and formal member, usable as a sentence adverb before the verb or alone as a firm, polite affirmative reply.

  certainly 是三者中最中性、最正式的词，可放在动词前作句子副词，也可单独使用作坚定、礼貌的肯定回答。

  The manager certainly appreciated the effort her team put in.

  经理确实很感激她的团队所付出的努力。

• sure

  Sure is the most informal, used mainly alone as a casual affirmative reply in speech, rarely placed before a verb in this sense.

  sure 是三者中最口语化的词，主要单独使用，作随意的肯定回答，在这个意义上很少放在动词前使用。

  "Can you send me the file?" "Sure, I will do it now."

  “你能把文件发给我吗？”“当然，我现在就做。”

• surely

  Surely often expresses the speaker's confident expectation or hope, sometimes seeking agreement, and can sound persuasive or slightly rhetorical rather than purely factual.

  surely 常用来表达说话人自信的预期或希望，有时是在寻求认同，语气可能带有说服性或修辞性，而不只是陈述事实。

  Surely you don't expect me to finish this by tomorrow?

  你该不会真的指望我明天之前完成这个吧？

Certainly, sure, and surely all mean 'of course'. Certainly is neutral and formal, used before a verb or alone. Surely often shows hope or expectation, mainly in questions. Sure is the most informal, usually a short answer alone. Sure meaning 'confident' is not compared here.

这三个词在这一“确实、当然”的意义上很接近，容易混淆，但语气和搭配不同：certainly最正式中性，可放在动词前也可单独作答；surely常带说话人的期待或修辞语气，多用于反问句；sure最口语化，通常单独用作随意的肯定回答。这里不比较 sure 作形容词表示“确信的”这一用法。