Different questions /verb tenses/

[bayan said]

Dear sir,
Thank you very much for answering my messages.I'll put my questions about past perfect off till I've finished all my questions.My questions this time are:
1- on T.V they sometimes say "where have you been?"and sometimes"where were you?"although the person may sometimes have come recently they ask the second question,why?
2- What's the difference between"I suppose","It's just possible","maybe",
"perhaps"&"probably"?please if you say that a word is formal or natural explain what you mean by this..
3- what's the difference between"collecting stamps"&"stamps collecting"
which one is right in BE?when do we use each?
4- What's the difference between"must","have to"&"be to"?
Thanks in advance..
All the best..

[Tdol]

1 Not much difference really- it depends on how recently they have returned.
2 'perhaps 'is less likely than 'probably' and 'it's just possible' is a very remote chance. 'I suppose' is a way of accepting that something is possible, especially when the speaker doesn't really want to.
3 It should be 'stamp collecting' and then I don't think there's much difference, though 'stamp collecting' sounds more like the name of the hobby to me and 'collecting stamps' describes the activity.
4 must/have to can often be used interchangeably, though 'have to' can be used to show that the authority comes not from the speaker, but from an external source. 'Be to' tends to be used in formal language and doesn't always have the idea of compulsion.