因为it做形式宾语。

We must finish it by tomorrow.我们必须在明天之前完成它。Without it, the results of this experiment will face failure again.没有它，这次的实验结果又会面临着失败。He found it hard to work.他发现工作很难。Where's my penIt was on my desk a minute ago.我的钢笔在哪里 一分钟前它还在我的桌子上。