Some sentences are simple, e.g.:
1. John loves Mary. 2. Mary loves John.
But, of course, you could combine those two simple sentences together to make one longer sentences, as in:
3. John loves Mary and Mary loves John.
The word 'clause' is used by some linguists (including us) to refer, in sentences like (3), to the major 'simple-sentence-like' constituent parts within the sentence (though, confusingly, some linguists also use the word 'sentence' for both the overall structure and the major constituent parts!). Thus, sentence (3) 'John loves Mary and Mary loves John' has two 'compound' sentences. 'But', 'or' and 'so' are also co-ordinating conjunctions.
Try inventing some simple sentences of your own and co-ordinating them with 'and', 'but', 'or' and 'so'.
Why is it odd to co-ordinate (1) and (2) with 'but'?
Why is it odd to co-ordinate 'Mary beats John' and 'John loves Mary' with 'and'?
Even though the above two co-ordinations are odd, you can still construe a context for each which 'normalises' them. But, if you do, you have to assume something odd about John. What?
Note that as you do all this, you are beginning to imagine a 'world' behind the sentences (a world which in these cases is a bit peculiar, and so more noticeable). This is the sort of mechanism which leads us to create fictional worlds in our heads when we read literature.
There are basically two kinds of overhearers in conversations:
Those who are 'listening in' secretly or unnoticed by those taking part in the conversation. They have their own viewpoint, of course, but it can't be taken into account by the speakers, or expressed, as they are keeping silent (though note that in a play overhearers often give asides to the audience, and so their viewpoints are expressed), and
'Liscensed overhearers'- e.g. those not speaking, not being addressed, but who are known to be listening (e.g. other people in the wider conversatyion, or children present while their parents are having a row!). Their viewpoints can clearly be taken into account.