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 can't 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 car's 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'?