When you say why someone does something, you are talking about purpose. English gives you a few tools for this: to, in order to and so as to before a verb, so that before a full clause, and for before a noun.
The pattern
I went to Target to buy a lamp.
She speaks slowly so that her students can follow.
We stopped for gas.
01To, in order to, so as to
The most common way to give a purpose is to + base verb. It answers "Why?" and it's what Americans use most in speech: I called to check on you.
In order to means the same thing but sounds more formal, so it's common in work emails and instructions. So as to is even more formal and mostly written. All three work best when the subject is the same in both parts of the sentence.
- I'm taking a night class to improve my English.
- In order to enter the building, visitors must check in at the front desk.
- Please arrive by 8:45 so as to allow time for security.
02A negative purpose
For a negative purpose, use in order not to or so as not to. Plain not to on its own usually sounds wrong as a purpose, so avoid "I left early not to miss the bus." In everyday speech, Americans usually choose so (that) + won't / wouldn't instead.
- We whispered so as not to wake my roommate.
- I wrote the date on my hand in order not to forget it.
- I set two alarms so that I wouldn't oversleep.
03So that + can, could, will, would
Use so that when the purpose is a full clause, especially when the subject changes: I'll text you the address so that you can find it. After so that, we usually use a modal: can or will when the main verb is present or future, could or would when it's past.
In speech, Americans often drop that: I saved the file so I can finish it at home.
- I'm printing the map so that we won't get lost.
- She moved closer to campus so that she could walk to class.
- Speak into the mic so everyone can hear you.
| Main verb | So that + ... |
|---|---|
| present or future | can, will, won't |
| past | could, would, wouldn't |
04For + noun, to + verb
Use for before a noun: I went out for coffee. Use to before a verb: I went out to get coffee. Never put for right before a base verb, and don't use for + -ing to say why a person did something.
For + -ing is fine when you describe what a thing is used for: This app is for tracking your expenses.
- We stopped in Austin for lunch.
- We stopped in Austin to have lunch.
- This brush is for cleaning the grill.
05Common mistakes
I came to the US for study English.
I came to the US to study English.
Before a verb, use to, not for.
I went to the bank for getting some cash.
I went to the bank to get some cash.
To say why a person did something, use to + verb, not for + -ing.
She repeated the address so that I can write it down.
She repeated the address so that I could write it down.
After a past main verb, use could or would after so that.
I left early for not missing the train.
I left early so as not to miss the train.
A negative purpose needs so as not to, in order not to, or so that + wouldn't.