The to particle is needed because the infinitive expresses purpose or reason: I'll do all I can to help you . The fact that can precedes to help is of no grammatical significance. The meaning is entirely different if a bare/plain infinitive is used after can: I can help you.
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Cool BreezeThere is nothing [that] I can do to help him.Please help me with this question.
vsureshI'm sorry I unable to understand the change in relation to the question I have asked.I'm afraid I don't understand your question. I just corrected the term I used in my original reply. There's no comma in the relative clauses, which means they are defining.