为什么要说the building being built is our library而不说the building built is our library

the building being built is our library正在建造中的
the building built is our library已经建造完工的

being built 是定语，表被动，修饰那个building

为 了强调进行时态所以用现在分词被动做定语。。

这是英语的语法.习惯.英语就这样.在他们眼里汉语也这样.