mostly为什么可以加be动词

mostly是个副词，副词可修饰形容词，而形容词可以放在be动词后面，即be mostly+形容词，例如：

1、The hard-line speech appears to be mostly for domestic consumption强硬路线的讲话似乎主要是说给国内人听的。

2、Indeed， our schools and universities are structured to mould pupils to be mostly obedient servants of rationality， and to develop outdated skills in interacting with outdated machines.实际上，学校和大学就是用来把学生塑造成理性的服从者，培养与过时机器互动的过时技能。

3、And my talk today will be mostly about these cognitive traps.我今天将主要谈论这些认知陷阱。