perposition

這條題目看似簡單, 但要明確講出它的分別的確不易 (當然係對我來講). 希望憑我所知, 可以俾到你多少頭緖啦!
你的問題: 為何 foot 要配合用 on, 而 bus 要配合用by呢?
當然, 這個係慣常用法, 亦都係指定用法.
先睇個by字: http://www.ldoceonline.com/dictionary/by_1
2 means/method used to say what means or method someone uses to do something: (係一個途徑, 透過佢我地可以完成一些事)


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You can reserve the tickets by phone.

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Send it by airmail.

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Some customers prefer to pay by cheque.

by car/train/bus/taxi etc (指「坐交通工具」)

圖片參考：http://www.ldoceonline.com/images/407:408M/entry/pronsentencea.gif
They travelled to Chicago by train.

by air/sea/land/road/rail etc

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All supplies are transported by air.

by doing something

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She earns her living by selling insurance.

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He was taken from his home by force.

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如果用on去配合bus其實係可以的, 但意思就係 「身處巴士上」, 而並非 「乘坐巴士」啦! 所以可能為咗要將兩者分開, 所以「乘坐巴士」就不能用on字啦

on bus: 指你的位置 (身處巴士上/內), 可以係一個靜止的狀態, 無論架巴士是否行駛, 只要你在上面, 都可以形容你係on bus, 未必代表你係由一個地方去緊另一個地方, 而by bus就不同, 係一個途徑, 即是含意比on bus更多, 你不單身處巴士內, 而且仲係由一個地方去緊另一個地方.
by bus: by係指你透過這些途徑去做一件事.


http://hk.dictionary.yahoo.com/search.html?s=by means of

用;以 by (即是係一個方法/ 途徑)
They succeeded by means of perseverance.
他們依靠堅忍不拔而獲得成功。
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on foot: 步行 (字面意思: 在你的腳上 ie. 你靠雙腳站著)

有一句慣用語
stand on your own (two) feet INFORMAL
to be able to provide all of the things you need for living without help from anyone else:

She'll have to get a job and learn to stand on her own two feet sooner or later. 自食其力 (字面意思: 靠自己對腳去站來).

希望你唔好覺得我離題. 我只係想值此說明, 某D字係慣常配合一起使用 (e.g. on + foot). 有時如果未能找出其微妙分別之前, 都要暫時死記先啦!


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peace