It's all about movement.
"Hier" implies a position that the speaker currently occupies, however, "hierher" implies a movement to that position from another place.
I.E.: "hier" is just "here" but "hierher" is "to here".
„Ich bin hier.“ - "I am here."
„Ich komme hierher.“ - "I come (to) here."
Keep in mind however, that:
- „Woher kommen Sie?“ means "Where do you come from?"
- „Wohin gehen Sie?“ means "Where are you going to?"
So, the difference lies within the focus; hierher focuses on moving to here from another place, whereas hierhin focuses on the target, namely to here, although in general they are used interchangeably, and there are regional preferences and so on.
It helps to remember that verbs involving movement, "gehen", "kommen" etc. that involve a location or destination require a direction in German, whereas if you occupy a position, then you don't.
- „Ich gehe dahin/dorthin.“ - "I am going there."
- „Sie ist da/dort.“ - "She is there."
- „Sie kommen hierher“ - "They are coming here".
- „Wir sind hier.“ - "We are here."
- „Gehst du hinein?“ - "Are you going in?"
- „Kommt er heraus?“ - "Is he coming out?"
Overall, in this context, the speaker is asking the recipient if they often come to here. It seems a bit strange in English, but that's just how it is in German; they tend to qualify a little more than we do in English. Hope this helps!
Well, actually, whence means "where from" and thence means "there from."
- Whence came the thing? Thence came the thing! (Where did the thing come from? The thing came from there!)
Whither means "where to," and thither means "there to."
- Wither shall we go? Thither shall we go! (Where shall we go? There shall we go!)
Not quite, "often" means that the thing in question is done frequently, while "usually" is used for something that is part of a routine, or something that is done more than half the time. If you usually do something, then you may often do that thing, but just because you often do something does not mean you usually do it, if that helps =P