The infinitive of purpose

A to-infinitive can be used to express purpose:

I'm calling to place an order for delivery.
Diya went to the door to open it.
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In order and so as can be used before a to-infinitive for emphasis in more formal styles:

He took a book with him in order to have something to read on the train.
The parties started negotiations so as to reach an agreement as soon as possible.
In order to attract a wider audience, we need to rethink our marketing strategy.

The negative is always in order not + to-infinitive or so as not + to-infinitive:

He tiptoed through the hall so as not to be heard.
In order not to lose time, we must act at once.

The infinitive of purpose can only be used if the doer of the action expressed by the infinitive is the same as the subject of the main clause. If the subjects are different, we can use so + that-clause with the present simple tense or with the modal verbs may, can, will, might, could or would:

Jerry works hard so that his family has everything they need.
I'll leave the door open so that you can come in.
Tina gave me a shopping list so that I wouldn't forget anything.

In order + that-clause is also possible in this case; however, it is more formal and less common. In the that-clause, we can use the modal verbs may, shall, might or should:

Our company does everything in order that all complaints may be dealt with fairly and effectively.

With come and go, we can use the infinitive of purpose with the verb in any present or past tense or with the gerund form of the verb:

I'm just coming to help.
We went to talk to a lawyer.
Have you thought of going to see a doctor?

But if come and go are used as infinitives or as imperatives, we use and instead of to:

I must go and check the heater.
He will come and dance with you.
Go and fetch a glass.