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Huck assumes Tom's identity because Tom's aunt mistakes him for Tom, and he goes along with it so that he can try and rescue Jim.
When Tom appears he pretends to be his own brother, Sid, and starts to complicate the situation with Jim.
Huck's plan for rescuing Jim is simple, Tom's is absurdly complicated.
Huck becomes passive when tom arrives and lets him take charge, he seems to lose any ability to act for himself.
Tom's plan is very cruel because he knows that Jim is really free, yet he pretends he is not just in order to be able to play a game of escpaing prisoners. He lets Jim go on thinking he is still a runaway slave, whereas in fact he is a free man. It is a monstrously cruel thing to do.