Can I use the conjunction "before" with the main clause in the past perfect continuous or the past continuous? And which is best?

Question

My textbooks suggest that the only correct option is the one in the past perfect continuous because the past perfect continuous means an action ("planning") which lasted up to the later moment which is in the past simple tense ("I went out"). The past continuous, however, ("I was planning") is wrong here because it is used when one wants to say that an action was happening at an exact point in time - like 5 pm or when Mother was making lunch.

Which of the following sentences is considered grammatically correct and why?

"What was I planning to do before I went out?"

Or

"What had I been planning to do before I went out?"

Answer 1

Both are grammatically correct. The first one reads as if, before going out, you were planning what to do during your excursion. The second suggests you were planning something unrelated, and broke off to go out. – Weather Vane

There isn't a much difference between a past perfect continuous tense and a past continuous tense. In fact, there isn't much difference between a past perfect and a past tense. Most times when learners think they "must" use a past perfect tense, the past tense is also correct.

In particular, it is correct to use the past tense to speak about a past action that occurred before another one.

I was planning to study before I went out.

I got dressed before I made my bed.

I ate my pudding before I started my main.

All of these could be expressed with a past perfect, but all are correct and idiomatic with the past tense.

What is the past perfect for then? Not much really. It can be used in some emphatic and contrastive senses. It can add a nuance, such as "the action continued up to the time and was interrupted." Or it can be used in backshifting reported speech.