The document discusses infinitives of purpose, which are used to say why we do something and take the form of verb + infinitive, such as "I went to the store to buy milk". It notes that "in order" can be added before the infinitive to sound more formal or varied, and must be included when the purpose is negative, as in "We were speaking quietly in order not to wake up Mr. Smith". Infinitives of purpose can also be used to give short answers to why questions by stating the purpose alone, like answering "Why did you go to the store?" with "To buy milk".