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>
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 ...".