I found some text, which has weird (for me) usage of auxiliary verb - do:
Although C# doesn't strictly speaking have a pre-processor, it does have conditional compilation symbols which can be used to affect compilation.
I cannot understand correctness of this phrase. I thought that correct variant is:
it has
"it does" is used here to draw attention to a potentially surprising, or perhaps contradictory part of the text.
The passage has said earlier that it doesn't have a pre-processor, but it points out that it has some other features which perform a similar function, and so "it does have ..." draws your attention to the "suprising" aspect of it.
It can be seen as a sort of companion to "Although" at the start: it has the form "Although <X>, it does <Y>", where Y contradicts X in some way, or is suprising given X.
You can use this in various different ways, eg
"I don't usually enjoy sports, but I do like golf."
"Sarah doesn't go out much, though she will always come to my parties."
You could just say "it has", like so:
Although C# doesn't strictly speaking have a pre-processor, it has conditional compilation symbols which can be used to affect compilation.
And this is fine, grammatically, but the original is better because the "it does" draws your attention to the surprising aspect.
Okay, I'm not a native English speaker like you, but I've been using English for a good amount of time.
I think when you add 'do/does' in front of a verb, you have two (or more) statements/ points that are contrary or not supportive to each other, and you want to emphasize one that is weaker than the other so the reader won't miss both and perceive what you have said more accurately. Then, the one that was emphasized becomes your point, and the another coexists with it.
A preceding 'do/does' often expects a 'but/although' to precede. Actually in my opinion, when you start a sentence with a 'but/although' and you use plain verbs it will be awkward to native speakers because they correspond to each other.
Example:
Nick: Amina, I've got the flowers for you. It cost a lot.
Amina: But they don't look good. Did you recycle it to not pay for it or what?
Nick: No, I didn't and it did cost it a lot.
I'd be my pleasure if it made sense to you. Good night :)
