介词和介词短语有什么区别，怎么判断是介词还是介词短语

简单的解释是
介词是一种用来表示词与词， 词与句之间的关系的词。

介词是一个单词，比如
in, at, on, over, under, about, above, across, with， 等等，等等
用法例子：
the bridge OVER the river
the man WITH the blue shirt

介词和其他代词或名词搭配形成介词短语。
介词短语是一个词组。

与介词一样，介词短语是用来表示词与词， 词与句之间的关系的词组。
如何判断是介词短语：

1. The book (on the table) in the English classroom is Barbara's book.
2. The girl (in the neighboring house) plays the flute every night.
3. Large blocks (of the hardest granite) formed the walls (of the new building).
4. The roads (of ancient Rome) connected the cities (of the empire).
5. I know that man (in the gray suit and the suede shoes).
上面括号内是介词短语，各个修饰前面的名词 book, girl, blocks, walls, roads, cities, man 。