English — Infinitive of purpose
To say why you do something, English uses to, in order to or so as to + the infinitive: I went out to buy milk. The negative is in order not to or so as not to.
How it works
The infinitive of purpose answers "why?". The plain to-infinitive is the most common (I went out to buy milk); in order to and so as to are more formal or emphatic (She left early in order to catch the train). For a negative purpose you must use in order not to or so as not to (I wrote it down so as not to forget).
Examples
| Structure | Example | Use |
|---|---|---|
| to + infinitive | I went out to buy milk. | Neutral, the most common way. |
| in order to + infinitive | She left early in order to catch the train. | More formal or emphatic. |
| so as to + infinitive | He spoke quietly so as to avoid waking the baby. | Formal; common before avoid/be. |
| in order not to + infinitive | I wrote it down in order not to forget. | Negative purpose. |
| so as not to + infinitive | We tiptoed so as not to wake them. | Negative purpose, more formal. |
| for + noun (not infinitive) | I went out for some milk. | Purpose named as a noun, not a verb. |
Good to know
Never use for before a purpose infinitive: not "I went out for to buy milk" — say I went out to buy milk. Use for only with a noun (for some milk) or an -ing form describing what a thing is used for (a knife for cutting bread). Also, don't put don't inside the infinitive: it is in order not to forget, not "in order to don't forget".