Question: I found a sentence in test.

> 
> Not until several years after a war has ended \_\_\_\_\_\_\_\_\_ to feel the severe psychological damage it can cause.
> 
> 
> 

(A) do many of its veterans begin  

(B) many of its veterans begin  

(C) and many of its veterans begin  

(D) many of its veterans beginning 

I chose B answer, but the correct answer (due to test) was A.

Could anybody explain why here we should use *do* clause?

Answer: 
This is a phenomenon called *negative inversion*: when a sentence starts with a negative adverbial like "never" or "not every day" or "not until several years after a war has ended", we get subject-auxiliary inversion that looks just like what you see in questions. So, for example, "you don't see that sort of thing every day" can be rephrased as "not every day **do you see** that sort of thing", and "I'll never help him again" can be rephrased as "never again **will I help** him".

Oddly enough, negative inversion also happens after some adverbials that are not obviously negative, or not completely negative; for example, it's also triggered after "rarely" and "only ...".
Well, the *real* reason is that the others sound wrong to a native English speaker.

As for the technical justification, what you are looking to do here is start what is called a [verb phrase](https://en.wikipedia.org/wiki/Verb_phrase). In English, a verb phrase is supposed to start with a verb. The words "many" and "and" are not verbs. "Do" is. 

In fact "do" is a [special verb](http://www.quickanddirtytips.com/education/grammar/the-verb-%E2%80%9Cdo%E2%80%9D-is-weirder-than-you-think) that can be used specifically in instances where you need to turn a phrase like this into a verb phrase.