What is meant by semantically negative and structurally negative sentences?

Sentences in English may be negated (made negative) by
negation markers or by sentence meaning. Negative
markers are no, not, neither, none, nor, etc.
Negative sentence meaning conveys negation through a variety of different means, for
example, by using negating words such as decline,
absent, renege, revoke, refuse, etc.

These two methods of
sentence negation are called structural negation and semantic
negation. Structural
negation is when a sentence is negated (made negative) through
negation markers: Al did not wish to see the
film. The negative marker not is placed after the modal
verb in the sentence of negation. These sentences of negation use a modal verb that
precedes the negative marker: did
not.

Semantic
negation is when a sentence is negated through
sentence meaning: Beatrice declined to be civil
and amiable to Benedic. This can be paraphrased as Beatrice was not
civil nor amiable to Benedic. This method of negation may be harder to
understand because the negation is contained in the vocabulary
rather than in easily recognized
markers.

Another means of semantic negation is through
ironic sentences that mean the opposite of what they
directly say, for example, That will teach you to go on blind dates
as paraphrased actually means, That will teach you NOT to go on blind
dates.