Why does "will" go before a noun in this sentence?

Question

Why does "will" go before the word "Twitter" while there's no question?

Under no circumstances will Twitter be liable in any way for any Content...

Answer 1

This putting of "will" before "Twitter" is an example of what is called "subject-auxiliary inversion", more specifically, "negative inversion".

The word "Twitter" is the subject (the key "actor" in the clause), the word "will" is an auxiliary verb.

Let's make a basic positive indicative sentence (that simply indicates something):

Twitter will be liable for some Content. (Subject + Auxiliary + Main verb)

Here, the auxiliary "will" goes together with the main verb "be", as auxiliaries usually do: "will be".

The simple negative form of this statement would be

Twitter will not be liable for any Content under any circumstances.

Here, we've added the negative word not between the auxiliary and the main verb ("will not be"), and added the adverbial "under any circumstances".

Note that there's the word "any" in the adverbial. Now we can make an "introductory negative" out of the adverbial. We take the negative word "not" away from its position betwen the auxiliary and the main verb, and place it in the adverbial, and move the adverbial ahead:

Under no circumstances Twitter will be liable for any Content. (not yet a valid sentence)

To make the negative meaning of the sentence more readable, we undertake a subject-auxiliary inversion, moving the auxiliary verb "will" closer to the "introductory negative". Thus we break the "will be" combination that reads like a positive statement on itself:

Under no circumstances will Twitter be liable for any content.

Now, with words "will" and "be" a little further away, and with "will" closer to "no", it's less likely that the reader will read "will be liable" as a positive statement.

This construction is used in legal English to give more force to the negation. It is also used in poetry, again to provide emphasis:

Am I a spy in the land of the living, that I should deliver men to Death?
Brother, the password and the plans of our city are safe with me; never through me shall you be overcome.

Here, "never through me" is an introductory negative adverbial that carries the negative word ("never"); the positions of the subject ("you") and the auxiliary ("shall") are inverted.

---

References

1. Negative Inversion - Wikipedia
2. Quirk et al, "A Comprehensive Grammar..", topics 3.24, "Inversion of subject and operator", and 18.24 (page 1382), "Subject-Operator inversion"
3. "Conscientious Objector" by Edna St. Vincent Millay

Answer 2

When I wanted to answer another question, I saw a helpful link about inversion in grammar. Here it is.

As user StoneyB once pointed out, inversion is almost disappearing from English usage. But I think this use will remain, at least in the exams. :)

Under no circumstances will Twitter be liable in any way for any Content...

This is not a question. It's a perfectly grammatical statement in which "Twitter" and "will" have been replaced in order to simply put emphasis on the meaning of the sentence. No other evil plans hide in this sentence . If it was me, I would have made it bold. That way it seemed way more emphatic. :)

Here's an approximately comprehensive tutorial and here is an English tense tm quiz. Hope I've helped.