A practical english grammar - Vyssaja skola 1978
The infinitive of purpose
Sound symbols
The infinitive can function quite often as an expression of purpose. This differs from the verb + infinitive patterns we have discussed earlier in that in the former there is a limited number of characteristic verbs, while with the infinitive of purpose any verb that makes sense can be used. Sometimes only a knowledge of idioms enables a speaker to tell which construction is which, because they are identical in form.
He wants to talk to my sister.
He stopped to talk to my sister. (Stop is not one of the characteristic verbs that are followed by the infinitive, and so the hearer knows that to talk expresses purpose.)
Here are some other examples:
I stood on the chair to change the light bulb.
Mary went downtown to buy a new dress.
The maid hung the clothes on the line to dry.
In case there is ambiguity, so as or in order may be used before the infinitive to express purpose.
Let’s begin, so as to finish earlier.
If one said “Let’s begin to finish earlier,” the sentence would sound like nonsense, because begin followed by an infinitive means that the infinitive names an action that the main subject starts to do.
I ask in order to defend my reputation.
This sentence means that the speaker is asking (for something) for the purpose of defending his reputation; if he had said “I ask to defend . . .” he would have meant that he is asking permission to defend his reputation.
The infinitive of purpose is used a great deal with nouns. In this case, it usually has a passive meaning.
I have a book to read.
There’s nothing to buy here.