”If he really wants to be the best in the world, as he loves to claim he is, he needs to improve his attitude.”
A possible problem with this sentence is the combining of "wants to be", which seems to refer to a future aspiration, with "is", which refers to a present state. This is not necessarily wrong, but there is a possibility that it may not properly express what the writer actually means. Apart from this, the sentence is fine.
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A possible problem with this sentence is the combining of "wants to be", which seems to refer to a future aspiration, with "is", which refers to a present state. This is not necessarily wrong, but there is a possibility that it may not properly express what the writer actually means. Apart from this, the sentence is fine.