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The problem is so severe to be overlooked.
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I think you mean this.

The problem is too severe to be overlooked.

This means we can't overlook it.

Clive

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ElmaAffirmative to negative

Aff: The problem is too severe [to be overlooked / to overlook].

Neg: The problem is not severe enough to [investigate / take a look at / pay attention to].

too [ADJ] to > not [ADJ] enough to

CJ