1. 有时候区别很小,常可互换。如:
the relation [relationship] between cause and effect 因果关系
the relation [relationship] between wages and prices 工资与价格的关系
What's the relation / relationship between Tom and LInda? 他们俩人是何关系?
2. 有时候区别很明显:relation 亲戚 / relationship 亲属关系