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。