用"fall down"造句如下：
Don't fall down from the stairs.
解釋：不要從樓梯上摔下來。
The house looked as if it was about to fall down.
解釋：那所房子看起來好像就要倒塌似的。
If the blocks are placed off-centre, they will fall down.
解釋：如果積木搭得中心錯位，就會倒塌。
Babies often fall down when they are learning to walk.
解釋：小兒學步時常會跌交。
The old lady fell down in the street and broke her leg.
解釋：老婦人在大街上摔倒了，並且摔斷了腿。
You must watch your step so as not to fall down.
解釋：你必須留心腳下才不會跌下去。
In autumn, the leaves fall down from the trees.
解釋：秋天，樹葉會從樹上掉下來。
注意："fall down"的意思根據語境有所不同，可以表示「摔倒、跌倒」，「（貨幣、貨幣價值等）下跌」，「（貨幣等）貶值」，也可以用於表示建築物等的「倒塌，坍塌」。在具體使用時，需要根據上下文來選擇合適的解釋。