Final answer:
The subject of the sentence is 'he.'
Step-by-step explanation:
In the sentence 'when the mother gives the visitor a hug, he begins to cry,' the subject is 'he.' The subject is the person or thing that performs the action described in the sentence. In this case, the mother gives the hug, so she is the one performing the action, while 'he' is the one who begins to cry.
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