In the following situation, does "very" or "so" sound more natural?
You usually go to bed early. Last night you went to bed very late (for you) and as a result you are very tired this morning.
Friend: You look tired this morning.
You: Yes, I'm not used to going to bed ____ late.
I answered "very", but the answer in the book (English Grammar in Use, Unit 61.2.4) is "so".
What is the difference between "so" and "very" in this context?
Using "so" would be in relation to something else, so it would be "so" because You are going to bed late in relation to or in contrast to your normal bedtime. Using "very" would just mean it's really late, but "so" helps the reader know that you don't usually go to bed this late. As a native speaker as well, "so" sounds more natural. "Very" means that it's really late, but "so" means its really late in contrast to your normal bedtime so it's more effective and gives more emphasis.
Both are good grammar, but only "so late" fits the context.
"So late", in this context, means "as late as that", so it's correct here because the sentence means, "I'm not used to going to bed as late as I did last night." Because of the "(for you)" part of the context, we have no idea what time they went to bed, so we don't know if it's later than people normally go to bed.
"Very late" would mean much later that most people normally consider late.
The person in this context went to bed later than they normally do, but nothing indicates whether they went to bed later than people in general normally do, so "very late" is incorrect.
