I mean using it for answers and questions that people can do without signing up.

If someone wants to have an account to follow his answers and questions, it's also possible,

Not sure if this can be configured from the Yaf options by an admin.

Many thanks.

Yes that is possible. You can assign access mask for every forum, to a specific role.

In Your case you need to set the access mask member to guest users

Yes thats also possible you can watch topics, and a user can set an option to enable email notification on every topic the user started or posted in.

Thank you.

You said: Yes that is possible. You can assign access mask for every forum, to a specific role.

How can I activate access mask for every forum? Can the users answer with a name? Or they will all appear as Guest?

When you create a forum or subforum you have do assign an access mask to each role. Take a look at the attached Picture

In the current version there appear as guest but in the next upcoming version a guest can enter a user name which will be shown to the post.