near可作表语、前置定语、状语,nearby可作定语（前置后置都ok）、状语,near by可作表语、后置定语、状语.前者可以表示时间、空间、程度等方面的接近,而后两者只能表示空间上的接近.因为现在说的是他们之间的关系,所以就只说表示空间的用法.
1作表语时,near与near by可互换.如
The shop is near （my home）.
=The shop is near by （my home）.
2作前置定语时,near通常用比较级或最高级,而nearby无比较级和最高级.如the nearest town ,the nearby town.
3作后置定语时,nearby可与near by互换,如
the town nearby=the town near by.
4作状语时,near可与nearby互换,如
She lives near=She lives nearby.
near有时也可与near by互换,如
There is a new theatre near our house.
=There is a new theatre near by our house.
因为是原创,所以只供参考.