[صلاح محمود محمد]

I think we should consider the difference in the following sentences.
I had been planning my summer holiday in Alexandria when my sister suddenly fell ill.


I had planned to spend the summer holiday in Alexandria when my sister suddenly fell ill.

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In the first example (without "to") : the meaning is; I'm thinking carefully about something I want to do to decide how and when I will do it, and the verb in this meaning is dynamic which means; it could be used in the continuous tenses.

In the second example(with "to") : the meaning is; I intend to do something, and the verb in this meaning is stative which means; it couldn't be used in the continuous tenses

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So, I think the correct answer is (had planned) but the examiner didn't intend it to be so, he just wanted (had been planning) which will be correct only if the verb is dynamic.
Secondly, I think students are not supposed to answer such questions, it is unfair.
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