他們一定發生了意外。

They must have had an accident.

Source: Tatoeba

Sentence Composition

1 他們 tāmen they; them
2 一定 yīdìng
  1. surely
  2. certainly
  3. necessarily
  4. fixed
  5. a certain (extent etc)
  6. given
  7. particular
  8. must
3 發生 fāshēng to happen; to occur; to take place; to break out
=4 了 le
  1. (completed action marker)
  2. (modal particle indicating change of state, situation now)
  3. (modal particle intensifying preceding clause)
=4 了 liǎo
  1. to finish
  2. (used with 得 dé or 不 bù after a verb to express (im)possibility, as in 忘不了 wàngbuliǎo "cannot forget")
  3. (literary) (usually followed by a negative such as 无 wú or 不 bù) completely (not); entirely (not); (not) in the least
  4. to understand clearly (variant of 了 liǎo)
=4 了 liǎo
  1. (of eyes) bright
  2. clear-sighted
  3. to understand clearly
=4 了 liào unofficial variant of 瞭 liào
5 意外 yìwài
  1. unexpected
  2. accident
  3. mishap

classifier: 個