🔹 S + had + been + V-ing→ She had been studying for two hours before the exam started.
🔹 S + had not (hadn’t) + been + V-ing→ They hadn’t been sleeping well before the trip.
🔹 Had + S + been + V-ing?→ Had you been waiting long before he arrived?
before, until, when (trong câu có 2 hành động quá khứ)
for + khoảng thời gian → for 2 hours
since + mốc thời gian → since 9 a.m.
how long (trong câu nghi vấn)
By the time..., already, just (ít gặp hơn)
Không dùng với động từ chỉ trạng thái (know, like, believe, understand...) → ❌ I had been knowing him for years. → ✅ I had known him for years.
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