How long had Dan been driving before he fell asleep and had an accident?

To answer this question, we need to understand the concept of past perfect tense and its usage in English grammar. The past perfect tense is used to describe an action that happened before another action in the past.

Usage of past perfect tense:
The past perfect tense is formed by using the auxiliary verb "had" followed by the past participle form of the main verb. It is used in the following situations:

1. When we want to show that one action happened before another action in the past.
2. When we want to express a completed action that happened before a specific point in the past.

Analysis of the given sentence:
The sentence "How long had Dan been driving before he fell asleep and had an accident?" is a complex sentence that combines the past perfect tense and the past simple tense.

Breaking down the sentence:
1. "How long" - This phrase indicates the duration of time.
2. "had Dan been driving" - This is the past perfect tense construction, indicating that Dan had been driving for a certain period of time.
3. "before he fell asleep and had an accident" - This is the past simple tense construction, indicating two actions that occurred after Dan had been driving.

Explanation of the answer:
The correct answer is "had Dan been driving" because it correctly uses the past perfect tense. The sentence is asking about the duration of time that Dan had been driving before he fell asleep and had an accident. By using the past perfect tense, we understand that the action of Dan driving happened before the actions of him falling asleep and having an accident.

In summary, the past perfect tense is used to indicate that an action happened before another action in the past. In the given sentence, "had Dan been driving" correctly represents the duration of time Dan had been driving before he fell asleep and had an accident.