weak没有第三人称单数形式，因为它是形容词。

adj. [经] 疲软的虚弱的无力的不牢固的

She was born with a weak heart.

她生来就有一个衰弱的心脏。

She was too weak to stand.

她虚弱得站都站不住。

She is still weak after her illness.

她病后仍然虚弱。

His weak voice was almost inaudible.

他声音微弱得几乎听不到。

He was a weak little mouse of a man.

他是个懦弱无能的人。