When to use do/does/did if the sentence is not a question?

[rte.sushil]

I was wondering when to use form of verb "to be" (do/does/did) in a sentence if it not a question.
E.g.
I did go there
He did see it.

According to me,
Is it like if some body ask a question "did u go there?" then in reply we say "yes i did "or "yes i did go there".
or
if we want to emphasize on the verb e.g. "He did love you" instead of "he loved you"

Question:-
Is there any rules in using the form "to be" in sentence (other than interrogative) ?

[jlucero]

Not sure if this was a mistype in your post, but be (am, is, was, were, being) and did (do, does, doing) are two different verb forms, so be careful that you don't get these two confused. Be = something is; did = performed an action. Unfortunately, these are two of the most versatile verbs in the English language and have many different usages that each one can incur. Rather than memorize all of them, understand that there are a few idiomatic usages of each that the GMAT might test- highlight any such instances you come across during your studying, but don't spend two weeks learning all of them.

One of the (many) reasons of using "do" is, as you say, provide emphasis on an action. For a longer list, I'm going to reference Mr. Google:

http://www.google.com/search?q=be+defin ... 40&bih=805

and "be":

http://www.google.com/search?q=be+defin ... 40&bih=805

[rte.sushil]

Only when a caterpillar has grown to the right size for pupation to take place does a special enzyme halt the production of juvenile hormone.

When i come across this sentence, i found it little unusual as we don't use in daily communications.
I would have said :
Only when a caterpillar has grown to the right size for pupation to take place then a special enzyme halts the production of juvenile hormone.

1.) Is my change correct? If so then we can write only....then sentences as in original example?
2.) Can you share some examples to understand such usage?
3.) What is the name of such usages<so that it is easy to search such examples >

Thanks in anticipation

[RonPurewal]

Only when a caterpillar has grown to the right size for pupation to take place does a special enzyme halt the production of juvenile hormone.

The GMAT will not test this kind of usage. I.e., you won't have to decide whether it's better than some other type of usage.
Your only job here is to realize that this construction is not wrong.
I.e., you just have to recognize it, so that you don't mistakenly eliminate it.

This is a style thing. The sentence is reversed to emphasize a certain part; there's no grammatical issue.

Only when you need money do you call me.
Grammatically, there's no difference between this sentence and "You call me only when you need money." The difference is that the speaker wants to place additional emphasis on "only when you need money""”i.e., to call the other person a leech/freeloader/user.

"Do call" is used because that's the only way to reverse the sentence like this.

If there's already a helping verb, then you don't have to change it.
You should go visit your ex-boyfriend only if you never want to see me again.
Only if you never want to see me again should you go visit your ex-boyfriend. (even more emphatic)

When i come across this sentence, i found it little unusual as we don't use in daily communications.

This type of thing is actually pretty common, if this sort of emphasis is desired.

[RonPurewal]

I would have said :
Only when a caterpillar has grown to the right size for pupation to take place then a special enzyme halts the production of juvenile hormone.

You can't use both "when" and "then" to describe the same temporal relationship.

2.) Can you share some examples to understand such usage?

See above.

3.) What is the name of such usages<so that it is easy to search such examples >

Sorry, I have no idea.
I really don't know any grammatical terms, beyond basic parts of speech (noun, verb, adjective, etc.)