1+2. No - a) being tall and thin is quite usual so why would you want to use a contrast adverbial? b) because in both cases the sentences are too short. As I said above, if short clauses are used, it's more natural to join them in writing by using a co-ordinating conjunction to make one longer sentence. In spoken language there are no sentences, but the ideas would be juxtaposed in the same way. So a more natural phrasing would be :
1. Mike is tall and thin.
2. A : I don't think Mary is the right person for the job. B : Well, it's true that she isn't very kind - but she's quite smart.
3. There's no contrast here - rusty is not the opposite of black. And if someone asked me if a knife was black, why would I a) repeat the question b) mention it was rusty?? It doesn't make sense without a context. The dialogue would go something like :
A : Is the knife you're looking for black?
B: What? The handle? No, it's brown. It's got a brown handle and a rusty blade.
Here, it's clear
why the speaker is giving these details - so that the listener has more chance of identifying the knife if she finds it.
3+ 4 These are clearly spoken contexts - and we don't speak in sentences but simply juxtapose or co-ordinate ideas - as in my example for (3) above. In addition, sentence connectors are used much less frequently except in more complex contexts. As above, when the proposition is very short and simple, juxtaposition or co-ordination is more usual. The dialogue in 4 might go something like :
A : Are oranges usually cheap here?
B : Usually, yes - but they're more expensive at the moment 'cos they're out of season.
To analyse language properly you must put it in a realistic context. This involves considering a multitude of factors such as :
- Who the participants are and their relationship
- Whether the discourse is spoken dialogue (short turns), spoken monologue (long turns) or written. All will have different features.
-
Why they are saying/writing what they are saying/writing
and a lot of other factors which are less important in your examples.
It was the lack of the last factor (why) that made it impossible to analyse your sentences without changing them.
But as I said above, you should never invent language in order to analyse it. Take your examples from a
concordancer.