The basic (declarative) sentence is:
He does not know.
You can turn this into an interrogative clause with subject-auxiliary inversion. Just switch he and does:
Does he not know?
You can optionally replace does not with doesn't in the original sentence, using the suffix -n't rather than the word not:
He doesn't know.
Now the auxiliary is the single word doesn't, so if we apply subject-auxiliary inversion, we get:
Doesn't he know?
Lastly, you can turn the original sentence into a question by using rising intonation at the end, which you indicate in writing by replacing the period . with a question mark ?:
He does not know?
However, this is only appropriate in certain situations, for example as an echo question—repeating what you heard as a question to express incredulity or to confirm that you heard the speaker correctly. Most (but not all) situations where this would be appropriate are informal, and as a result "He does not know?" sounds somewhat unusual. Instead, you can use the contracted form:
He doesn't know?
But most of the time the interrogative form is more appropriate.
- Who did she come?
It is very important to check whether the verb to come is a transitive verb (which doesn't require a preposition) or intransitive verb (which does require a preposition). For example, if you change the interrogative sentence to a declarative one, it will be
She came who.
It doesn't make any sense. It should be
She came with whom.
Therefore, you need the preposition with in the sentence. When "whom" is placed at the beginning as an interrogative pronoun, "who" is more broadly used than "whom".
- How long did they stay [for]? Where did Jack go [to]?
You need to understand that "how long" and "where" are interrogative adverbs, not interrogative pronouns. Therefore, you don't need to use the prepositions. The same rule applies to "home" as in
He went home. *He went to home
You don't need to use "to" in the second sentence above as "home" is an adverb. If you replace home with "where", it would be easier to understand how it works.
He went where. Where did he go?
- When did you call me?
"When" is an interrogative adverb which doesn't require a preposition.
You need to differentiate interrogative pronouns such as who, which, what, etc. from interrogative adverbs such as how long, when, where, how, etc.
From time to time, you can hear some native English speakers ask "Where are you at?" or "Where are you going to?" The prepositions at and to are not absolutely necessary, however, they could be used. You can read “Where are you now at?” — grammatically correct? and is “Where are you going to?” correct to understand how they work. They are a few exceptions to the rule.
Best Answer
There isn't anything ungrammatical about either sentence here. But, the social interaction between the two speakers seems to be wrong. There is a problem with the pragmatics.
It's important to note, though, this might just be because we haven't heard the rest of the conversation. This could be a perfectly normal exchange. We'll look at that a bit later.
Here's why the response above is strange. Grammatically, we can identify different types of clause. The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002, p. 853) identifies five main types of clause:
These different types of clause have different grammatical properties. Notice, for example, that in the interrogative clauses there is subject auxiliary inversion are you instead of you are. Also with the imperative, we see a grammatical sentence that has no subject, unlike the in the other examples. The exclamative has a wh- word fronted to the beginning of the clause, but unlike the interrogative clauses, there is no subject auxiliary inversion. We see you are, not are you.
Now, we tend to associate these different types of clause with specific types of communicative acts. So we associate declarative clauses with people making statements. We associate interrogatives with questions, in particular questions asking for information. We associate exclamatives with exclamations and we associate imperative clauses with people giving directives (telling people what to do).
However, we don't always use imperatives when we want to make a directive. For example look at the following sentence:
The sentence above uses a declarative clause. However, it is being used as a directive. The speaker is telling the listener what to do. We can also use declarative clauses to make questions:
A: Lady Gaga fainted!
B: Lady Gaga did what?
The questions in the examples above are declarative clauses, not interrogative ones (notice that there is no subject auxiliary inversion in the Lady Gaga question, even though it uses the interrogative word what).
In the original Poster's example, we see an interrogative clause:
However, negative interrogative clauses using why are usually not questions they are usually directives. They are usually suggestions. The Original Poster's interrogative would normally be interpreted similarly to:
Because this is not a question, because this is really a suggestion, the response is wrong:
We understand this phrase as an infinitive of purpose. It tells us why we did or did not do something. But the speaker is not (in the normal reading) asking for information. They do not want to know why we do something. They are making a suggestion. So a correct response to Original Poster's first sentence is something like:
This is a way of agreeing to the suggestion that we go a different way.
Garden Paths
Sometimes a sentence can have more than one meaning. If we do not have a clear context for the sentence, we will just give it the most likely reading. With negative why-interrogatives, we will normally read the sentence as a suggestion, unless there is some reason to believe that the action described did happen, does already happen, or will definitely happen. If we assume that the action we are reading about does happen, or will (continue to) happen, we are likely to read the sentence as an information question:
If we assume that the speaker expects to continue going this way, we won't interpret the interrogatives in these examples as suggestions. They are questions seeking information. In this situation the correct response would be:
The reason that we decide that the response in the Original Poster's example is bad, is that without any other information, we automatically assume that the first sentence is a suggestion. Having already interpreted it as a suggestion, the response seems inappropriate. We have seen though, that in reality it is not necessarily wrong.
The purpose of this question in the textbook is to see if students understand this method of making suggestions. It is testing their interpretation of Why don't we-sentences.
When ambiguous sentences like this lead us to one particular interpretation even though they (might) have another meaning in this context, we sometimes call them "garden path" sentences. We could argue that the answer to the Original Poster's example was correct. Because we did not have any context, the first sentence was a garden path sentence. We are just misunderstanding the question. However, if you want to pass an English test, it is important that you can recognise interrogatives used as suggestions.
Garden path sentences are often used in jokes. Here is one of my favourites:
Note:
Notice that this feature of why-interrogatives is not shared by negative interrogative clauses which use different wh- words. For example, consider:
Neither of these would be interpreted as a suggestion. Also notice that to save money is a good answer to the second question here.