drop in的例子：Drop in to see us any time.随便什么时候顺便来看看我们.I' d like to drop in and see you sometime next week.我想在下周什么时候顺便来看看你.It's improper to drop in on him so casually.这样冒冒失失去找他可不好.Oh! I just drop in on him for a chat.哦,我只是顺便来和他聊聊天drop by的例子：I have to drop by the bank to get some money.我得到银行去取一下钱Would you drop by when you are in town?你来城里就来玩好吗?I'll drop by later to check on you.等一下我会顺道去看看你.Only very close friends drop by unannounced.只有非常亲密的朋友才可能不请自来,从上面的对比看,两个词组的用法上的区别是drop in后面可以跟on,后接拜访的人,而drop by一般不这么使用.另外,drop in含有临时生出主意造访的意思,造访未经事先安排.而drop by没有明确这层含义