A man can do what he wills, but he cannot will what he wills
means that a man can do whatever it is he likes. He cannot make
other thing do what he wants them to do just because he wants them
to do it. He doesn't have that power.
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Insha' Allah in Arabic means: God willing, or if God wills. It
is added at the end of a sentence where one hopes that the said
statement will/should happen if God wills.