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What is the difference between:

  1. "How it works?"

and

  1. "How does it work?"
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How it works is a phrasal noun. How does it work? is an interrogative sentence (question). The former is commonly used as a heading and there is no question mark at the end. –  Kris Feb 20 at 10:20

3 Answers 3

up vote 9 down vote accepted

SHORT ANSWER:
Your first example, how it works, is a free relative clause which cannot be used as a question. Your second, How does it work?, is an ordinary question.

LONGER ANSWER:
What I’m going to call an “interrogative phrase” (IP) is a sort of ‘variable’ which stands for an unknown ‘value’. The IP is headed by a word which defines the ‘type’ of value for which it stands: who, what, which define the type as nominal, how, where, why define the type as adverbial, and the construction what ... do defines the type as a verbal. The IP may also be followed by additional terms which further restrict the type—how many or what sort of machine.

Two sorts of clause employ IPs: questions and free relative clauses. In both sorts of clause the IP represents a constituent of an ordinary declarative clause, ‘moved’ to the beginning of the clause from the place where the constituent would normally stand:
IP Movement

But the two sorts of clause play very different roles, and have different syntactic structures:

  1. A question asks the hearer to supply the value for the variable named by the IP and defined by the remainder of the clause. A question is an independent clause—it can stand on its own.

    The first syntactic rule is that first constituent1 after the IP must be a tensed verb. If the IP stands for the Subject of the clause, nothing has to move, because the IP and the verb are already in these positions:
    Q-SubjIP

    But if the IP stands for some other constituent, the tensed verb must fall before the Subject, and a second rule comes into play: the tensed verb must be an Auxiliary. (Grammarians call this subject-auxiliary inversion.) If the ‘canonical’ statement version of the clause does not have an Auxiliary verb, the appropriate form of DO is pressed into service. (Grammarians call this DO-support.)
    Q-AuxInv Thus the proper form for a question using how is this, with both subject-auxiliary inversion and DO-support:  Q-How 1Note that an adjunct—a non-essential syntactic component—is allowed to fall between the IP and the tensed verb: Who recently wrote an app?

  2. A free relative clause does not ask for the value of the IP but designates it—hearers may fill it in from their own knowledge, but an actual value is not required.

    A free relative clause does not call for either inversion or DO-support, regardless of which constituent the IP stands for. After the IP at the beginning, the ‘natural’ order of a declarative sentence is maintained, Subject-Verb-Objects/Complements; the only thing that signals that this is not an ordinary declarative sentence is that something is missing where a constituent was ‘replaced’ by the IP and ‘moved’ to the front.

    A free relative clause is always a dependent clause—it is embedded inside a ‘head’ clause and acts as a noun phrase. In these three examples, the head clause is in ordinary black type; the free relative acts as Direct Object, as Subject, and as the Object of the preposition about.
      (Since DO-support is not in play here the tensed verbs are not distinguished.)FR-Syn How it works, then, with the subject before the verb, is a free relative clause. Here are some examples of how it might be used:  FR-How

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I'm a bit confused on what the difference is between your interrogative words and relative words. That is, when there is a difference between them and when they are the same words. It seems that in a prototypical interrogative main clause, e.g. "What did Sue eat?", that the word "what" is an interrogative word but not a relative word. But for embedded questions and relative clauses/phrases, e.g. "Tom knows what Sue ate", then, the "what" word is a relative word, is that right? -- So does that mean that, in your grammar, there are no interrogative words in subordinate clauses/phrases? –  F.E. Oct 15 at 20:00
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In the example "Tom knows what Sue ate", let's assume that Sue had eaten a ham sandwich, and so, that means that Tom knows the answer to the question 'What did Sue eat?' which is that Tom knows that Sue had eaten a 'ham sandwich' -- But it does not mean that Tom personally knew that 'ham sandwich', not unless that ham had been Tom's pet pig earlier before it was slaughtered and cured and made into a delightful sandwich (which an attempted fused-relative interpretation of that example might be: "Tom knows that which Sue ate"). –  F.E. Oct 15 at 20:27
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Here's an example that helps show that a fused-relative is a different beast from that of a subordinate interrogative clause. From a 2005 textbook by Huddleston and Pullum, A Student's Introduction to English Grammar, page 192: "[22.iii] What she wrote is unclear. [ambiguous: relative or interrogative]" . . . "Unclear, however, licenses both an interrogative and an NP subject, and [iii] can be interpreted in either way. The fused relative interpretation is 'That which she wrote is unclear' -- a letter or report, perhaps. (cont.) –  F.E. Oct 17 at 5:18
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Many expressions when taken in isolation could have a shape that could be of different categories. Consider the expression "the bagels you can have", which could be a NP in "Those are the bagels you can have", or it could be a clause with its object preposed as in A: "Are those cupcakes for sale?" B: "No, they're a special order. But the bagels you can have." (last example borrowed from CGEL, pg 1369). It's the matrix clause or the context that is needed to know how to interpret an expression. Often a similar situation arises with subordinate interrogative clauses and fused-relatives. –  F.E. Oct 17 at 20:13
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@F.E. Certainly. And what's in the box may represent either an independent interrogative clause or a dependent free relative. But there are both syntactic and phonological reasons for distinguishing those two categories; I see none for distinguishing free relatives from 'embedded questions'. Do you categorize the school as two different sorts of NP in the two interpretations of "I ran for the school"? –  StoneyB Oct 18 at 0:42

'How it works' used in an interrogative phrase is something you may not find very commonly spoken amongst native speakers.
Such a sentence construction can be correctly used as an assertive statement (for instance, newspaper headings). Another example is:

Rob knows how it works.

But then again, it may be a part of an interrogative sentence, as in:

Will you tell me how it works?

Here, 'how it works' is correct because it isn't a direct part of the question being asked. The question asked is, "Will you tell me _?" 'How it works' is just that part of the sentence which goes into the blank, as an assertive phrase.

'How does it work' is what you'd say while asking somebody about the way it works. This construction is used in interrogative sentences.

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I'll correct Maulik's second answer:

So now you know how it works, don't you?
How does it work? Do you know?

You see that he sets the expressions in context, which is always very important.

If you're using a sentence fragment rather than a sentence proper in your first example (say as a section heading), you'd drop the question mark (and probably the period):

How it works

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