表示建房子和桥梁通常是要 complete 的，对不对？

The bridge was supposed to have been completed by 1992.

如果用 finish 呢？就算错吗？

如果把complete换成  finish，也是可以的，不过用complete会更好，因此表示完美结束意味，finish没有这个意味，指表示完成。

(1)complete侧重表示做完或完成某工作，特别是一项任务，或者把某工作圆满结束。complete后接名词往往更具体些，如“建筑物”、“工程”、“书籍/著作”等.

The house was completed five moths ago. 

这座房子是5个月前竣工的。

A few cars, however, completed the race. 

有几辆车跑完了比赛的全程。

They said that they could complete the work in two months and a half.   
他们说他们能在两个半月内完成了这项工作。

The workers haven't completed the house yet.   
工人们盖房子还没有完工。

We completed the work five days ahead of time.   
我们提前五天完成了工作。

It took the builders of the water tower two months to complete the task.   
修建水塔的工人花了两个月的时间完成这项任务。

She completed her set of dishes by buying the cups and saucers.   
她买了茶杯和茶碟，把她的一套茶具配齐了。

The bridge was completed by the end of 1994.  

这座桥是在1994年底建成的。

Two days later a map was completed.  

两天后,地图画好了。

Just as breakfast was completed there was a knock at the door.  

早饭刚刚吃完,外面就有人敲门。

The project will be completed by the end of this month.  

该工程到月底完工。

I need one more stamp before my collection is completed.  

再有一张邮票,我的收藏便完整了。

(2)finish表示过去某个时候着手做的事已经做完，或者表示对已经做完的事进行精密加工 ，后面可接名词或动名词，往往带有反面的意味，如“用完”、“吃完”等。

As soon as he had finished, Jenny told him she wanted to go home. 

他一讲完，珍妮就告诉他说她想回家。

This machine part needs surface finishing. 

这个机械零件需要精密加工。

They finished the bottle of beer between them.   
他们两人把一瓶啤酒喝完了。

The boy finished the cake just in a few bits.   
这孩子几口就把蛋糕吃完了。

If the rain holds off for a few days more, they can finish the planting.   
如果这场雨晚几天再下,他们的种植任务就可以完成了。

If you had not interfered,I should have finished my work by now.   
要不是你打扰的话,我现在早已完成我的。