by、 through、 via 的區別
1. by：在旁邊，從旁邊，到旁邊（強調從旁邊經過或路過）
（1）pass/go by 是指從旁邊經過或走過；
（2）something is done by somebody 是指某事在某人身邊（旁邊）被做完，借指某事由某人完成；
（3）by 9 o’clock 是指在9點鐘附近（或左右），因為當有人問你要時間表的時候，基本上是希望你在某個時間點之前完成，如果在之後完成則說明那個時間點定得沒有多大意義。
（4）by train 等短語是指在火車近旁或者火車在自己近旁，借指自己在火車上。
在歸納出並抓住了 by 的本義之後，我們就會發現 by 其實並不是用來表示「經過、通過」的，它的這一意思是借用出來的，是用法所致。或者說，即便用 by 表示「經過、通過」的意思，也是「從旁邊」經過或路過，所有的抽象用法都是由此衍生出來的。
2. through：貫穿，穿過，透過，通過（強調從內部穿透）
through一詞比較簡單，它指的是從事物的內部穿透過去，比如說，光線透過眼鏡的鏡片進入我們的視網膜，河流穿過峽谷或城鎮，某人闖過某片樹林或叢林等等。注意：這個內部可以是完全封閉的（比如身體），也可以是部分封閉的（比如地表或峽谷），甚至可以是開放的（比如某一扇牆壁或門板）。through 的核心意思是從中穿透或貫穿。
through同時也可以可以被定義為「由於」 (as a result of)。例: The company lost the order through production delays. 該公司由於生產延誤，失去了訂單。
3. via：經由，途徑，從某處（強調通過中間的媒介達成目的）
via一詞似乎用得較少，日常生活中我們經常喜歡用by而不用via，比如要表達「發郵件」或「以郵件的方式」，基本上多數人第一個想到的就是 by email，其實這時候用介詞 by 的話，嚴格說起來是非正式的用法，正式的用法是用via。*
*請注意，多數母語人士仍會使用 by email。
前面已經分析過 by 的本義，很明顯 by 表示“通過某種方式或途徑”的用法（如by train / by means of等）是借用或引申推廣出來的，但by在表示該意思的時候有個前提：主體自己要跟隨工具在一起，這是by的本義（在附件、在旁邊）所決定的，或者說，主體和工具不能發生分離（這是by與via的本質區別）。然而，這只是一種解釋。對於母語人士來說，by 和 via 在這種情況下是可以互換的。
而via表達的意思是“經由某個人、某個區域、某個工具達到某個目的”，這裡的“人/區域/工具”都是中間媒介，它們起著傳遞、連接等作用，這時候主體跟工具是註定要分離的。也就說：
（1）send someone a note via email 是指經由email的傳遞給某人發個便條通知某人某事情；
（2）travel from A to C via B 是指從A地出發經由B地前往C地；
（3）broadcast via satellite 是指經由（或通過）衛星進行廣播。
資料來源: http://hi.baidu.com/heartsoft2008/item/0d5a0c12f56249ce39cb30dd
English explanation:
The word “by" is most often used, and can mean the following:
1 near someone or something; at the side of someone or something
-The telephone is by the window.
2 used to show who or what does, creates, or causes something
-He was knocked down by a bus.
3 used for showing how or in what way something is done
-The house is heated by gas.
-to travel by boat/bus/car/plane
to travel by air/land/sea
4. used to show the degree or amount of something
The bullet missed him by two inches. House prices went up by 10%.
More uses: http://oaadonline.oxfordlearnersdictionaries.com/dictionary/by
“Through" generally means to go from one end of something/place to the other.
1. from one end or side of something or someone to the other
-The bullet went straight through him.Her knees had gone through
-The sand ran through (= between) my fingers.
2. see, hear, etc. through something to see, hear, etc. something from the other side of an object or a substance
-I couldn’t hear their conversation through the wall.
3. past a barrier, stage, or test
-Go through this gate, and you’ll see the house on your left.
4. (also informal thru) until, and including
We’ll be in New York Tuesday through Friday.
5. by means of; because of
-You can achieve success through hard work/by working hard.
More info: http://oaadonline.oxfordlearnersdictionaries.com/dictionary/through
“Via" emphasizes by someone or something.
1. through a place
-We flew home via Dubai.
(This use is synonymous with “through/thru."
2. by means of a particular person, system, etc.
I heard about the sale via Jane.The news program came to us via satellite.
(This use is synonymous with “by.")
source: http://oaadonline.oxfordlearnersdictionaries.com/dictionary/via
So at times, the words are interchangeable, but they all emphasize different things.
Sources:
1. oaadonline.oxfordlearnersdictionaries.com
2. http://www.differencebetween.com/difference-between-through-and-vs-via/#ixzz2VK59sm4n
3. http://hi.baidu.com/heartsoft2008/item/0d5a0c12f56249ce39cb30dd
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