for作介詞，表示時間、距離,意為“計、達”。如:
I usually do the running for an hour in the morning. 我早晨通常跑步壹小時。
We will stay there for two days. 我們將在那裏逗留兩天。
介詞by可以用來表示方位、時間、手段等：
壹、by+地點名詞。表方位，意為：“在…旁邊”。如：by the lake/river/tree/window/door等。
She searched the top of the hill and stopped to rest on a big rock by the side of the path.(NMET90)她搜索了山頂，然後停下來在路邊的壹塊大石頭上休息。
After supper she would sit down by the fire,sometimes for as long as an hour,thinking of her young and happy days.(2003上海·春）晚飯後，她常坐在火旁，有時長達壹個小時地回憶她那青春、快樂的年代。
註：有時可表:“從…旁經過”,多與動詞go/walk/pass等連用。
As we boys were rushing towards the playground,Jim slipped by the table.(NMET92完形）當我們這些男生沖向操場時，吉姆從桌旁溜過。
二、by+時間名詞。意為：“到…時（已發生某事），此時謂語多用完成時；最晚、不遲於…，在…之前”。如：by now/then/this time/next Friday/the end of/three o`clock等。
By the end of last year,another new gymnasium had been completed.(2003上海·春）到去年年底，又有壹座新體育館峻工了。
The train leaves at 6:00 pm.So I have to be at the station by 5:40 pm at the latest.(NMET97)火車下午六點開，所以最晚我得五點四十趕到車站。
三、by+名詞。可用來表方法、方式、手段等。
1、by+the+可數的時間、長度、重量等名詞。意為：“按…計算，按…買(賣)”。如：by the pound/ton/yard/meter/dozen/bale/day/month等。
Cleaning women in big cities usually get paid by the hour.(NMET98)城市裏的女清潔工通常按小時獲得報酬。
In the market,tobacco is sold by the bale,eggs by the dozen and salt by the pound.在市場裏，煙葉按捆賣，雞蛋按打賣，鹽按磅賣。
2、by+表示時間、長度、重量等總稱的不可數名詞（名詞前不加冠詞）。意為：“按…計算，按…買(賣)”。如：by time/volume/length/weight/height/depth/width/area等。
As we all know,the freight of the luggage is charged by weight.眾所周知，行李的運費是按重量計算的。
3、by+交通工具、交通方式名詞（名詞前不加冠詞，不變復數）。意為：“通過…，由…，乘…”。如by train/rail/tube/taxi/bus/truck/bike/boat/plane;by land/road/sea/water/air等。（on foot)
It takes a long time to go there by train;it`s quicker by road.(或by bus)(NMET93)乘火車去那兒要花很長時間，公路比較快。
“I usually go there by train”.“Why not try going by boat(或water)for a change?”(NMET92)“我常乘火車去那兒。”“為何不嘗試壹下坐船呢？”
4、by+抽象名詞或具有抽象意義的普通名詞（名詞前不加冠詞，不變復數）。意為：“靠…，通過…，由…所致”。如：by skill/determination/practice/diligence/inference/chance/accident;by mail/letter/radio/fax/telephone/telegraph/hand/machine等。
Although he had been searching in the wrong area most of the time,he found it by logic,not by luck.(NMET2002閱讀）雖然他曾大部分時間壹直在錯誤的地方尋找，但他還是靠邏輯推理而非運氣，找到了它。
I made a coat with my own hands.It was made by hand,not by machine. 我用雙手織了壹件外套。它是用手工織的，而不是機器織的。
5、by+ 地點或工具等具體名詞。表路線、途徑，意為：“通過（某物、某地），取道…”。如:by a stone bridge/the back door/the freeway/country roads等。
The man entered the room by the back door.那人通過後門進入了房間。
6、by+動詞—ing.意為：“通過…，靠…，憑…”。如：by waiting/practing/begging/working/imitating等。
He gained his wealth by printing works of famous writers.(NMET95)他通過印刷著名作家的作品而獲得了巨大的財富。
Our bodies are strengthened by taking exercise. Similarly,our minds are developed by learning.(2000上海)身體是靠鍛煉強壯的，同樣，大腦是靠學習開發的。
四、by+數量詞。
1、表升降、增減的程度。如：by one-fifth/20 percent/￡3/two feet等。
With production up by 60%,the company has had another excellent year.(NMET2000)產量上升了百分之六十，這家公司今年的業績極好。
The demand for organic food is increasing by about one third every year.(NMET2002閱讀)對綠色食品的需求每年在增長大約三分之壹。
2、表示距離和面積、體積中的尺寸及乘除法中的運算。如：by 2cm/6 inches/four feet/nine yards/a hair`s breadth等。
Luckily,the bullet narrowly missed the captain by an inch.(2002上海）幸運地是，那顆子彈差壹英寸就擊中了上尉。
The bed is three meters wide by two meters long,and can fit at least four at one time.(NMET2001閱讀)那張床三米寬，兩米長，壹次至少可以睡四個人。
五、by+身體部位名詞或衣服名詞。表示被抓住身體/衣物的某壹部分，常與動詞catch/seize/take/hold/grab等連用。如：by the hair/hand/arm/nose/horns;by the collar/lape/sleeve等。
The policeman caught the thief by the arm.警察抓住了小偷的胳膊。
The soldier grabbed the child by the collar.那個士兵揪住了孩子的衣領。
六、by的常見短語：
by means of/by way of/by the way/one by one/step by step/side by side/shoulder by shoulder/by oneself/by birth/by society/take…by surprise/learn…by heart/what do you mean by …/by my watch等。