Look at these two examples:
Why did Dolores try to flood the forge?
Why Dolores tried to flood the forge?
Is the second example grammatically wrong as the helping verb is not used before the subject in the interrogative sentence? What is the exact rule for constructing this type of interrogative sentences?
There is one subject and two verbs! If you want to form an interrogative sentence, a question, you must have a helping verb or an auxiliary one. That said, the first sentence is correct
Why did Dolores try to flood the forge?
The second sentence may not be considered as interrogative because it does not have the helping verb though it has another verb try.
However, the second sentence can be used in certain context provided it should not be called a question or interrogative sentence. Rather, it would be a normal sentence.
I don't know why Dolores tried to flood the forge.
A must-to-read reference is here.
The second one ("Why Dolores tried...?") is grammatically incorrect.
Making questions in English is tricky! We always use the helping verb "to do" when creating a question, like
The cat eats fish.
Does the cat eat fish?
unless the original verb was to be:
Ellen is tall.
Is Ellen tall?
or it was a modal:
Ray can swim.
Can Ray swim?
or the verb was in a perfect tense:
They have eaten.
Have they eaten?
Sometimes in British English to have is treated the same way.
He has a ticket.
Has he a ticket? (British)
Does he have a ticket? (US)
If we're asking about the subject of a sentence, using who, which, or what, we don't use "to do":
The captain gave a speech.
Who gave a speech?The fifth president resigned.
Which president resigned?A book fell off the shelf.
What fell off the shelf?
But for the statement "Dolores tried to flood the forge", the main verb is not to be, it's not in the perfect tense, to try is not a modal, and we're not asking a who/which/what question about the subject, so it follows the normal rule, and we have to use "to do": "Why did Dolores try...?"
