تفاوت بین USE TO با USED TO چیست؟

In this video, Alisha answers 6 questions.
- Which one is correct: I want rest or I want to take rest?
- What is the difference between use to and used to in fast speech? I cant recognize the difference in the pronunciation.
- Which word do you prefer using as an American? America, the United States, the United States of America, the US, the USA, and the States?
- Which one is correct and why? The car keys? The keys of car? The cars keys?
- Is there any difference if we use yet at the beginning or at the end of a sentence?
- How can I distinguish in which from at which?