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There have been several reasons why~~

have comes due to a plural noun "reasons".. but I remember some cases of inconsistency: There has ~~ + a plural noun... or There is + a plural noun..

and one more thing.. A + B is........ / An apple and pear are fruits... but I found A + B doesn`t always have are but is as a main verb of a sentence.

Thanks.
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What exactly is your question, Moon? Can you produce some more specific examples that concern you? Compound nouns that function as single concepts, for instance, often take singular verbs: Bacon and eggs is my favorite breakfast.
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Thank your for your answer.

I should put more effort when making a question.

Ye,, your answer for compound nouns that function as singular concepts is my question. When, in particualr, using with there has and there is..

Since I got to know the concept of a compound noun, my question has solved.

for instance, often take singular verbs: Bacon and eggs is my favorite breakfast.

You said "often" in this sentence; because Bacon and eggs may not come out together as one menu or something in a eatery don`t they?

What other compound nouns are there ?

Thanks
There are plenty; I doubt there is a list available; it often depends on the speaker's conception: Catching spiders and eating them is a lot of fun.