the teacher said "we had online classes last week"
`->` The teacher said they had had online classes the week before.
- Last week `->` The week before
- Thì QKD `->` Thì QKHT
- S + say(s)/said hoặc tell/told + (that) + S + V lùi thì
- Thì QKHT: S + had + VpII + ...
- Vì VpII của have là had nên ở thì QKHT là had had.