Hello!

The correct answer is "could not attend", as it means "was not able to attend". When could means "be able to" or "be allowed to", could is followed by a simple infinitive. More information at http://www.polseguera.org/advanced_english_grammar/modal_verbs.php

Could have + past participle is used for possibility in the past. More information at http://www.polseguera.org/advanced_english_grammar/modal_verbs1.php

Examples:
I could swim like a fish when I was five. (ability)
She could have gone to visit her grandfather. Let's ring him to see if she's there.
You could have taken the lift. Now it is working again. (Why didn't you take it?)
You should not have entered that area alone. It is very dangerous. You could have been robbed.
Margaret could not have had a car accident last night. She was ill in bed. (That is, it is impossible that Margaret had a car accident last night, as she was at home.)
I could not read when I was two. I was too young. (ability, the same as "I was not able to read when I was two")
I could not arrive home late when I was a teenager. My parents wanted me to be at home at dusk. (permission, the same as "I was not allowed to arrive home late")

Best regards,
Mike

Hi
i finally found a sentence which is neither conditional nor past tense of will have pp.

Paul would have considered her company on this climb a benediction.

Would you please tell me what kind of structure ( would have pp) we have here. What does this sentence mean?

Thanks

Hello!

If we say "Under different circumstances, Paul would have considered her company on this climb a benediction", we mean that if there had been different circumstances, he would have considered her company on this climb a benediction.

Best regards,
Mike