The conjugation "had" is the past tense of to have, and is used in the past perfect (had done) tense as well as the past perfect progressive tense (had been doing).
Examples :
I just HAD to answer THIS question!
"I've been had!", exclaimed the fraud victim.
Which dessert would you rather have had?
No one knew what he had been writing.
i do not know how to use embalming in a sentence. (there is the sentence)
So- you are asking when to use 'when' in a sentence. When you are asking how to use when in a question, you are already using when in a sentence, because a question actually is a sentence. I like to use when in a sentence whenever I like.
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Use it in a big sentence.
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