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An interrogative adjective is a word which both modifies a noun and indicates a question. The words "which" and "what" are used as interrogative adjectives. Whether these words serve as adjectives or merely pronouns, however, depends on whether they modify a noun or stand on their own.
Just as interrogative sentences are questions, interrogative adjectives indicate a question. When a sentence begins with an interrogative adjective, the reader knows a question will be asked. For example, in both "Which car is hers?" and "What river is that?" the adjectives indicate a question is about to be asked.
More importantly, like any adjective, interrogative adjectives modify a noun. Unlike most adjectives, however, the interrogative variety are more limited in scope than their general counterparts. This limitation is partly due to the few interrogative adjectives that exist. With only two options, the resulting scope is rather narrow.
Additionally, although adjectives may add detail and clarification, interrogative adjectives just clarify. For example, in the sentences "Which car is yours?" and "The blue car is mine," the phrases "which car" and "the blue car" both clarify there is a specific car that is in question. Only the phrase "the blue car," however, describes that specific car in any way.
In addition to modifying nouns, an interrogative adjective can modify a noun phrase. A noun phrase is group of words that acts like one noun. For example, in the sentence "Which road should I take?," "road should I" is the noun phrase, "which" acts as the interrogative adjective, and "take" is the verb.
"Which" and "what" are not exclusively interrogative adjectives, however. Depending on their placement in a given sentence, they can be pronouns. Simply rearranging the word order in a sentence may change the adjective to a pronoun. For example, in the sentence "Which answer is correct?," the word "which" is functioning as an interrogative adjective because it is modifying the word "answer." In the sentence, "Which is the correct answer?," however, "which" is no longer modifying "answer," "correct" is now the modifier, so "which" becomes a pronoun.
To determine whether "which" or "what" are pronouns or interrogative adjectives, a reader must first look at the word or phrase that comes directly after the word in question. If the word following "which," for example, is a noun, "which" is most likely an interrogative adjective. On the other hand, if the word following "which" is a verb, "which" is probably serving as a pronoun.
@anon303025: Welcome to the wonderful, confusing world of English grammar. I have a degree in it and my hubby has four years of Latin under his belt. We talked this one over for 10 minutes, and finally came to the conclusion that the reason the last sentence is incorrect is because "which" is not, in fact, acting as an adjective in that sentence. Even though the basic structure is identical, in that sentence, "which" is actually a *noun.* It's trying to be an adjective here, but it doesn't work as one. An adjective has to modify (or describe) something. In that sentence, strange as it may seem, "which" is acting as a noun. It's not modifying or describing another noun.
"I don't know which car is mine." works because "which" modifies "car." You could even get away with saying, "I don't know which car mine is," because "which" is clearly the adjective modifying "car."
Even "I don't know which is my car" works because there's an understood noun after "which." "I don't known which (one) is my car" or "I don't know which (car) is my car." It has something to modify, even if it's understood.
Because "which" in particular can wear so many hats, it's kind of a different situation. "How," "when" and "where" all act as interrogative adjectives, but in your sentences, they are adverbs of degree, time and location, respectively. They're not being adjectives. "Who" is a pronoun, but refers to the brother. You'd think "which" is a pronoun in that sentence, but no, in that sentence, it's used as a noun trying to act like an adjective, which is why it doesn't work.
As I said, welcome to the wonderfully confusing world of English grammar.
I am an ALT in Japan and ran across this sentence structure (being taught in the school) that didn't seem right to me.
"I don't know which my desk is." I sat and worked around the sentence and concluded that
"I don't know which is my desk," and "I don't know which desk is mine" both sounded right, but the first one sounded wrong.
I began to experiment with interrogative words and in the following list, all but the last sounded right.
I don’t know who my brother is.
I don’t know what your problem is.
I don’t know where your store is.
I don’t know how your car works.
I don’t know when your party is.
I don’t know
which my car is.
However, I have been having trouble finding any explanation for this online. Perhaps the grammar is correct but rarely used, or perhaps it is wrong. If so, why? I was wondering if anyone here would be willing to explain?
@dfoster85 - Your pronoun tip is right on! Grammar books tend to make everything sound really hard, but common-sense tips like that really make it easier to grasp.
I'm an English teacher and I actually had to look up your question about "whose." I see your point that it seems to be an adjective asking a question. But in English, "whose" is considered to be a *possessive" adjective, like "his" and "our," not an interrogative adjective.
Maybe that's because "whose" is not always a question word. You can also use it in a declarative sentence, like "The girl whose purse spilled was late for class."
Another good way to tell if "Which" or "what" is a pronoun is that if it's a pronoun, it will make sense to replace it with another noun. In the example above, you could say "Number three is the correct answer."
But I have a question. Couldn't "whose" also be an interrogative adjective, as in "Whose book is this?"