stop by/in（顺便）拜访；串门

to visit a person or place for a short time, usually when you are going somewhere else

come by（顺路）串门；拜访

to visit a place for a short time, often when you are going somewhere else

look in on someone串门；顺路拜访

to visit a person for a short time, usually when you are on your way somewhere else

这三个表达都有串门的意思，但一般指顺路拜访。

例句：

I stopped by her place on my way home yesterday.

我昨天回家的时候，顺路去她家串了个门。

drop in/ drop by上门拜访；串门

come round串门；拜访

drop in和come round也是串门的意思，但这两个表达没有强调顺路，既可以指顺路串个门，也可以表示专程拜访。

例句：

Come round tomorrow and we will play Chinese chess.

明天来串串门吧，我们一起下象棋。