burn down, burn up, burn out
bum down 指“烧为平地,烧毁”;也指“火力减弱”。
The house was burnt down in an hour.
房子一小时被烧为平地。
bum up 有“烧旺”之意,也可指“消耗掉”。
You'd better put more wood on the fire to make it burn up.
你往火上添着柴,让它烧旺。
bum out 指“火熄灭”。
The fire had burnt out before I returned.
在我回来前,火已经熄灭了。