I had asked this question on Bio.SE
Why are fearful stimuli more powerful at night?
Why fearful stimuli are more powerful at night?
Which one is grammatically correct and please explain the reason.
Why are fearful stimuli more poweful at night?
The use of the linking verb "are" is in the right place.
When you start a direct question with the wh-word "why" to ask about reasons or explanations, the why is always followed by be as a main verb + subject or an auxiliary + subject + main verb.
Why is he here?
Why are you angry?
Why are you doing it?
Why did he speak to him?
Why will you go there?
As for the sentence "Why fearful stimuli are more poweful at night?, it's not incorrect altogether. In fact, it's part of an indirect question that's more polite and formal. The word order in the indirect question is the same as in a normal statement. You can rephrase this question as follows:
Could you tell me why fearful stimuli are more poweful at night?
Tell me why fearful stimuli are more powerful at night.
When we make a question with question words like "what","where","why","when" and other ones, provided they are not the questions to the subject of the sentence, we put the copula before the subject. So the first question is correct.
Why(a question word) are (a copula) fearful stimuli (a subject )more powerful at night?
A copula used before or after a subject depends on each situation.
If we want to make question, a copula must be used before subject. For example: What are you doing now?
If we want to use it as a noun phrase, a copula should be put after subject. For example: That is what we are thinking now. I don't know why you did that.
Back to your question, this sentence is correct: Why are fearful stimuli more powerful at night?