They were best friends in school, but after graduation, they drifted apart.

drifted 是动词 drift 的过去式，drift 本意是"漂流、飘移"，引申为"逐渐改变位置或状态"。 apart 是副词，意思是"分开地、分离地"。 drifted apart 表示"逐渐疏远"，形容人际关系慢慢变得不再亲密的过程。