最近在教一個初中學生需要學習的文法課題時，發現即使是高中生，也會錯誤使用，以致詞不達意，所以我決定在這期的專欄中詳細講解一次，希望各位second language learner（第二語言學習者）能有更清晰的理解。
在英文句子裡，expressing purpose（表達目的）時，我們可以用so that、in order that、so as to、so as not to、in order to、in order not to來描述做一件事情的目的，寫的時候要多注意文法，首先就是留意so與so that / in order that的分別。
So用以表達因果
So that / in order that：與so的使用不同，so通常用以表達因果，如： 「I want to lose weight so I do exercise regularly.（因為我想減重，所以我恒常地做運動）」；「 I'm obese so I need to eat less. （因為我過肥，所以我需要減少進食。）」
So在這兩句中表達了事情的結果︰想減重做運動 / 過肥減少進食，so後使用的是一個clause（I do exercise regularly / I need to eat less）。
雖然so that / in order that 後也是一個clause，但表達的卻是目的而不是結果：「I do exercise regularly so that / in order that I can lose weight（我恒常地做運動，目的是為了能夠減重。）」
So that / in order that 通常會配以can / could / will / would使用，又例如「I told my mom to wake me up in the morning so that / in order that I wouldn't be late for school.（我告知媽媽早上要喚我起床，為了上學不會遲到。）」目的：上學不會遲到。
So as to跟動詞
So as to / in order to：同樣是用以表達purpose（目的），但文法上與so that / in order that的使用不同，so as to / in order to後跟的不是一個clause而是一個verb（動詞），並且要是 to-infinitive，例如：「I do exercise regularly so as to / in order to lose weight.」與之前表達的目的一樣，恒常地做運動是為了減重，但這次不需要寫完整的clause，verb也是用base form，不作任何改變。
如要表達negative的意思，則需加上not使用，例如：「I do exercise regularly so as not to / in order not to gain weight.（我恒常地做運動，為了不增加體重。）」
Revision exercises
Join the two sentences.
1. I stayed at school until 9pm. I wanted to finish my English project.（in order that）
2. I organized a party. I wanted to celebrate my mom's birthday.（so as to）
3. I avoid eating junk food. I don't want to become obese.（in order not to）
4. I have a part-time job. I need to earn extra money.（so that）
Answers
1. I stayed at school until 9pm in order that I could finish my English project.
2. I organized a party so as to celebrate my mom's birthday.
3. I avoid eating junk food in order not to become obese.
4. I have a part-time job so that I can earn extra money.
■程韻
作者簡介：英國文學碩士，在男校任教英語的女教師，愛好文學和動物。
隔星期五見報