drop in的例子：
Drop in to see us any time.
隨便什么時(shí)候順便來看看我們.
I' d like to drop in and see you sometime next week.
我想在下周什么時(shí)候順便來看看你.
It's improper to drop in on him so casually.
這樣冒冒失失去找他可不好.
Oh!I just drop in on him for a chat.
哦,我只是順便來和他聊聊天
drop by的例子：
I have to drop by the bank to get some money.
我得到銀行去取一下錢
Would you drop by when you are in town?
你來城里就來玩好嗎?
I'll drop by later to check on you.
等一下我會(huì)順道去看看你.
Only very close friends drop by unannounced.
只有非常親密的朋友才可能不請(qǐng)自來,
　　從上面的對(duì)比看,兩個(gè)詞組的用法上的區(qū)別是drop in后面可以跟on,后接拜訪的人,而drop by一般不這么使用.
　　另外,drop in含有臨時(shí)生出主意造訪的意思,造訪未經(jīng)事先安排.而drop by沒有明確這層含義