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In the hand of a typical it-cleft structure, the focus is on the complement of it-clause.

1. It was David who called.

2. It was impossible to say hello to everyone.

However, in another hand of an anticipatory structure, the focus is at the end because it enables a subject to appear at the end of a clause.

3. It was wrong paying $25 a week.

For sentence 2 & 3, There is a clash in each since it-cleft structure is the same as anticipatory structure.

Does the focus is on the adjectives or the rear subjects?

Thank you.
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Creating focus and moving the focus toward the end of the sentence are the same process, if I understand your question aright.

Focus:

1. David
2. say hello
3. paying $25
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To say hello to everyone was impossible.

Paying $25 a week was wrong.

How about changing the structure into a sentence of raised subject?

Is the focus/emphasis on the raised subject or the adjective?

Thank you.
English emphasis comes at the end. The beginning is just an introduction to the topic.