I heard someone saying "what is (something) stand for?" and another question format "how is this happen?"
Are these questions grammatically correct?
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You probably misheard your sentences. They are grammatically incorrect. Both is should be does since both stand and happen are verbs, not adjectives or participles.
What does NHL stand for?
* What is NHL stand for?
If we turn these two questions into statements, we'll get:
NHL stands for the National Hockey League.
* NHL is stand for the National Hockey League.
Again, since stand is a verb, only the first sentence is correct.
In fast speech, I think what does might indeed sound somewhat like what is (and vice versa), especially in American English, where a T between two vowel sounds is often pronounced more like a soft D.