Mr Sujit Kumar Roy (Noida, West Bengal) has sent a long list of questions. I don’t know whether I can answer all of them here. Let me make a beginning.
1. We use in to mean after instead of using the latter, e.g. He will come in two days.’  But He has come after two days.  Why?
‘After’ can mean ‘time as measured from a certain point onwards.’  The point can be in the past, in the present or in the future. But in can only measure time from the present.  So we can say either ‘He will come in two days’ time’ or ‘He will come after two days.’ In both cases here time is measured from the moment of speaking.
But in He came after two days obviously the time frame here is in the past. We were expecting him on the 15th but he came after two days - on the 17th.  Cf. Also: We reach Delhi on the 17th.  After two days we leave for Haridwar. You can’t use in here.
In the examples given above (in the last para) the speaker is speaking now and time is measured from a point in the past (first sentence) and from a point in the future (second sentence). The speaker can also locate himself in the past and recount a series of events as they happened.  The use of in and after will be on the same lines as described above.
It was the 10th of June.  I was expecting Kishore.  He came rather late in the evening.  He said that in another two weeks’ time he would pay off the mortgage.  But after six months I found the mortgage hadn’t yet been cleared.
2. Prepositions with committee
When you are appointed a member of a committee, you are on the committee.  So in relation to the committee, you are on it.
In respect of the individuals constituting the committee, you are a member of the committee. Further, any matter taken up by the committee is supposed to be discussed in committee. (Not: in the committee!)
The same use of prepositions is found with staff. A person teaching in the department of English, say, in a university, is a member of staff there; he is on the staff. (Notice the absence of the article in the first phrase. A member of staff is on the staff!).  I have chosen a university department to illustrate the point.  It could be a school, college, institute, etc.
The word staff is not limited in usage to teaching departments.  It can be used of a group of people in other areas also (a factory, a workshop, etc.).  In a factory, for example, shop floor workers are distinguished from ‘staff’ (who hold more responsible, administrative positions). The armed forces use the word staff wit different meanings.
3.  resign
This verb can be used    transitively or intransitively.  Sheila has resigned her job at the call centre. Dr Gupta was forced to resign his vice-chancellorship at the Patna University.  These are transitive uses. The object directly follows the verb.  In the intransitive uses resign is usually followed by from. Dr Ramani has resigned from the committee. Dr Nagabhushan was once at NASA. He resigned from that organisation and set up a research institute in Bangalore.
To simplify matters, you can say: resign from a body, organisation, etc; resign a post.
3. Arjun (e-mail) wants to know whether a sentence like:  Who does not know Gandhi? should be classified as an exclamatory sentence.
No, it is not an exclamatory sentence.  An exclamatory sentence can be paraphrased as a declarative sentence with am adjective in the predicate modified by an intensifier How strange=It is very strange or a predicate noun modified by an intensifying adjective What a shame=It is /was very shameful. But Who does not know Gandhi? can only be paraphrased as Every one knows Gandhi.  It is a rhetorical question.
A rhetorical question does not expect an answer.  It is only an emphatic way of saying something. But it has the structure of a question.  
It begins with a wh-word, has a question mark at the end and has subject - verb inversion (as in a question).  What could he do? (=He could do nothing).  Note the subject – verb inversion. In Who does not know Gandhi? there is no inversion because it is a question on the subject.  Questions on the subject show no inversion. Ravi came.  Who came?

The writer can be contacted at ksyadurajan@yahoo.com