His house burnt down. This is an affrimative statement about an event that happened once in the past, therefore we use the simple past tense of the verb burn = burnt. His house has burnt down [present perfect tense, and it is usually followed by another statement related to time or the number of similar events that have occurred in the past, such as: again, for the first time]. His house had burnt down. [past perfect tense, and it is usually followed by another statement related to another event which ocurred before the most recent event, for example: His house had burnt down before he had a chance to buy fire insurance on the house. His house was burnt down. [this is the passive verb form/aspect which means that the action (burning) was caused by someone or something. Usually we would include the cause of the house burning, for example: The house was burnt down by two young boys. example: The house was burnt down because/when it was struck by lightning.

His house burnt down - we don't know caused this, maybe it was an accident. His house was burnt down - BY somebody; somebody did this but we don't know who. His house had been burnt down - again BY somebody but this time, because of the tense, (past perfect) it happened before another event in the past. e.g. His house had been burnt down when he arrived back from his holiday