1. What do you have to do in an emergency?
2. What do you have to do in emergency?
(Which one is correct? Are both OK? Do we have to use 'a'?)
#1 is fine. #2 is not used.
Yes, we use "a" ("an") because in this sentence only ONE emergency is being referred to.
Both sentences are grammatically correct, but they have slightly different meanings.
1. "What do you have to do in an emergency?" suggests that the speaker is asking about the specific actions that need to be taken during any emergency situation.
2. "What do you have to do in emergency?" is grammatically correct but sounds a bit more unusual in everyday conversation. Generally, we use "an" before the noun "emergency" because it refers to a singular, unspecified emergency situation. The indefinite article "a" is needed to make the sentence grammatically complete.
So, in most cases, it is more common and grammatically sound to use "a" before "emergency" in this context.