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: 
When a sentence starts with 'Not until', as well as many other phrases, the grammar needed is Inversion, which means using the interrogative form of whatever tense the verb is in. 'begin' is in Simple Present, so 'do' is required. Here are some examples:

Not until John apologizes, will I go out with him again.

Not until the teacher explained it again, did I understand.

Not until I've been working for ten years, will I have saved up enough money to pay off my student loan.

Not until the temperature falls to O degrees C does water turn to ice.

There is a similar question at :[Not until ( sentence ) + do (sentence)](https://english.stackexchange.com/questions/231539/not-until-sentence-do-sentence)

but I believe I've explained it slightly differently.

"Not till I got home did I realise my wallet was missing."

(Cambridge Dictionaries Online)

<http://dictionary.cambridge.org/grammar/british-grammar/inversion>
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.