چگونه به درستی از Must have استفاده کنیم؟

In this video, Alisha answers 7 questions.
- Whats the difference between: is that how it is?, is that how it works?, thats not what it says, thats not how it works ?
- Sometimes I see a couple of sentences like :your dad must have had it for at least 2 years. Why they do like have had? Whats the rules of this?
- What is the different between Astronomy and Astrology?
- My name is Afeef, I have been studying Pharmacy, and my question is How to make English speaking skill in home alone??
- What is the deference between its up to you and youre up to ?
- What does sunglasses mean? and What do sunglasses mean? Sunglasses is a plural noun, should we use DO instead of DOES?
- What does a spirit animal mean? I dont think we have that kind of question in my country. Also, what are some possible answers?