Sentences may also be identified as topic-comment structures in the following situations:
(1) When a modal verb is present:
Yī gè rén yīng gāi xué yī mén wài yǔ。
One should learn a foreign language.
Duàn liàn néng zēng qiáng rén de tǐ zhì。
Exercise can improve one’s physique.
(2) When the sentence particle 了 le is incorporated (see Unit 8):
Hái zǐ shàng xué qù le。
The child has gone to school.
Tài yáng chū lái le。
The sun has come out.
(3) When a passive notion is implied (without the use of 被 bèi structure):
Yī fú dōu xǐ gàn jìng le。
The clothes have all been washed.
Fàn zhǔ hǎo le。
The meal is ready. (lit. The food has been cooked.)
(4) When a verb is unmodified or unmarked for aspect and indicates a permanent characteristic:
Cats eat fish.
Māo chī yú。
Tiě ài shēng xiù。
Iron rusts easily. (lit. Iron is apt to rust.)
Lǚ yóu zēng jìn jiàn wén。
Travelling broadens knowledge.
Tā xǐ huān dǎ lán qiú。
He likes playing basketball.
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.