I have few questions about Twitter, answer please?

[NHK]

1. You just '@' them, or direct message them.
2. I'm pretty sure you will see the replies to someone else's tweet only if you are following that person. So if I write something on twitter, and five people respond to that tweet, the only way you will see it in your time line is if you are following me, or one of those five people.
3. Usually you receive notification in your email that so-and-so is following you if you have it in your settings for twitter to send that update to you.
4. You block them.
5. Only the people who follow you will be able to see your tweets if you set your profile to private