一开始，我以为他病了。

At first, I thought they were sick.

Source: Tatoeba

Sentence Composition

1 一开始 yīkāishǐ initially; originally; at first; in the beginning
2 我 wǒ
  1. I
  2. me
  3. my
3 以为 yǐwéi to think; to believe (often with the implication that the belief is mistaken – unless referring to one's own current belief)
4 他 tā
  1. (third-person singular) (since the early 20th century, usu. male) he; him; his
  2. (bound form) other; another; some other (as in 他日 tārì and 他人 tārén)
5 病 bìng
  1. illness
  2. disease
  3. to fall ill
  4. defect

classifier: 場
=6 了 le
  1. (completed action marker)
  2. (modal particle indicating change of state, situation now)
  3. (modal particle intensifying preceding clause)
=6 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)
=6 了 liǎo
  1. (of eyes) bright
  2. clear-sighted
  3. to understand clearly
=6 了 liào unofficial variant of 瞭 liào