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。
