必定 and 万万: surely will / by no means
【高29】【高42】副词 / adverbs (语气)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
必定 expresses certainty: a prediction or an inference that the situation makes inevitable, "surely, bound to, without fail". It goes before the verb (often before 会 or 要): 他必定会来, 这样下去必定要出问题. It can precede a negation: 必定不会. It goes naturally with a condition set by 如果 / 若是 / 倘若. The sentence says what will happen or what follows from the facts, never an order.
万万 is a heavy intensifier for refusing, forbidding or ruling out. It almost always goes with a negation: 万万不能, 万万不可, 万万干不得, or 万万没想到 ("I never dreamed"). You find it in warnings, flat refusals and accounts of a surprise.
The neutral 一定 can also promise or command (你一定要来); 必定, more written, only predicts or infers. For a long historical trend, see 势必 / 必将 (hsk6-g28).
Examples
他必定会来的，你放心吧。
Tā bìdìng huì lái de, nǐ fàngxīn ba.He will definitely come, don't worry.
这样下去，公司必定要出问题。
Zhèyàng xiàqu, gōngsī bìdìng yào chū wèntí.If this goes on, the company is bound to run into trouble.
我万万没想到，他当面就拒绝了我们。
Wǒ wànwàn méi xiǎngdào, tā dāngmiàn jiù jùjué le wǒmen.I never imagined he would turn us down to our faces.
万万不能开着煤气出门。
Wànwàn bù néng kāi zhe méiqì chūmén.Whatever you do, never leave the house with the gas on.
触犯法律的事情是万万干不得的。
Chùfàn fǎlǜ de shìqing shì wànwàn gàn bu dé de.Breaking the law is something one must never do.
若是有个太太照顾着他，他的生活必定不会那么乱七八糟了。
Ruòshì yǒu ge tàitai zhàogù zhe tā, tā de shēnghuó bìdìng bú huì nàme luànqībāzāo le.If he had a wife to look after him, his life certainly would not be such a mess.
Common mistakes
✗ 万万要小心。 → ✓ 万万不能大意 (万万 goes with a negation: 不能, 不可, 没想到).
✗ 你必定要早点睡。 → ✓ 你一定要早点睡 (必定 predicts or infers; an order or a request takes 一定要 or 必须).