Forum: ell.stackexchange.com
Tags: <verbs><verb-agreement>
Title: Can ‘is’ and ‘are’ be used in the same sentence? ‘The conclusion is both are harmful.’
Question: The conclusion is both are harmful.   My friends told me that you can not use 'is' and 'are' in the same sentence.

Answers:
sents: text: When in doubt, I find it helpful to simplify the sentence.
text: Consider these: The conclusion is uncertain.
text: The conclusion is final.
text: The conclusion is X. Clearly, whatever the conclusion is, it's singular and needs a singular verb.
text: Now let's look at what the actual conclusion is: Both are harmful.
text: Again, clearly, "both" refers to two things and thus requires a plural verb, "are".
text: You could check that by replacing both: Cigarettes and gun battles are harmful.
text: Angry dragons and mean dogs are harmful.
text: Now, let's take that last conclusion example --
text: The conclusion is X. -- and replace X with what it actually is:
text: The conclusion is [both are harmful].
text: When you look at it that way, it becomes clear that the sentence as originally written is indeed correct.
text: My conclusion is that your friends are wrong.
answer_details: author: ell.stackexchange.com/users/54582/Roger Sinasohn
score: 55
sents: text: No.
text: No, no, no.
text: Send your friends back a grade.
text: Conclusion is singular.
text: Use is .
text: Both is plural.
text: Use are .
answer_details: author: ell.stackexchange.com/users/None/
score: 3
sents: text: In relative , noun or adverbial clauses we often face these kinds of sentences.
text: E.g
text: “Who I am is not important.
answer_details: author: ell.stackexchange.com/users/111986/Cgrhan
score: 2
sents: text: It all depends on the way you define what sentence means in your context.
text: If you mean the string between two periods, then yes, a sentence can have an arbitrary number of conjugated verbs, and hence is and are in one sentence are allowed.
text: For example: The weather is fine and all people are happy.
text: If you consider a sentence to be the part containing subject and conjugated verb (or an infinitive sentence), then it is not possible.
text: Then, the example above must be considered as two sentences that are contactenated.
text: The example in your question is really a sentence and its subclause - so it is one sentence in the first sense but two sentences in the latter sense.
text: For better readability, there should be a that in between: The conclusion is that both are harmful.
answer_details: author: ell.stackexchange.com/users/24336/rexkogitans
score: 1
sents: text: You have two ideas together.
text: One is that the conclusion is right.
text: The other is that both A and B are harmful.
text: You are putting these into one sentence and finding it troublesome.
text: You are also rushing by with spoken English and leaving out an implied word that explains things.
text: The conclusion is that both are purple.
answer_details: author: ell.stackexchange.com/users/None/
score: 1
sents: text: No reason they can't be used in the same sentence, but for the sentence you provided it's incorrect.
text: The context of the question is "can you use both or only one", so "both" is a solution and therefore: "
text: The conclusion is [the use of] both is harmful."
text: In the original sentence, "The conclusion is both are harmful.
text: ", this is implying that both words "is" and "are" are independently harmful, and they are not both harmful.
answer_details: author: ell.stackexchange.com/users/None/
score: 0

Summary of the answers:
If you mean the string between two periods, then yes, a sentence can have an arbitrary number of conjugated verbs, and hence  is  and  are  in one sentence are allowed .   For better readability, there should be a  that  in between:   The conclusion is that both are harmful.   
“Is" and "are" can be used in the same sentence. However, adding "that" may make the sentence clearer.