Why can I contract *across* a word, skipping the word in the middle? [duplicate]

Solution 1:

Even though the standard term for these combinations of an auxiliary + -n't is "contractions", grammatically they act like single, indivisible words*; rather than like two words slurred together (though that was evidently their historical origin). This single word is a auxiliary, so it goes in the normal position for an auxiliary in an interrogative sentence: after the question-word and before the subject.

Why wouldn't it be valid? Question word - auxiliary - subject - [rest of sentence]

When you use "would not," on the other hand, you have two words: the auxiliary "would", and the distinct word "not" (a word that doesn't fit very well into the general part-of-speech categories). The auxiliary again goes into the normal position for an auxiliary. The word "not" has its own, different rules for where it goes in a sentence.

Why would it not be valid? Question word - auxiliary - subject - [rest of sentence]

*In fact, some linguists think the negative suffix in these words is best analyzed as an inflection in modern English, like the suffix "-s" used in third person.

Solution 2:

I explain it this way:

Why wouldn't it be valid?

Because this is a question, the subject "it" goes directly after the helping verb "wouldn't" and before the verb "be". Notice the difference between a question and a statement:

Why would it be valid?

It would be valid.

When you make a contraction, multiple words become one word. "Would not" becomes "wouldn't". In a question, the subject—"it" in this case—would still appear after the helping verb "wouldn't" and before the verb "be".

Solution 3:

[It appears to me now that Joseph's answer is better than what I said below. At any rate, the key point is that "not" does not hop over the intervening "it" as part of the process of contraction.]

It's because the contraction is obligatory, in this case. For understanding the syntax, you can regard "Why wouldn't it be valid" as equivalent to *"Why would not it be valid". The only reason the latter example is ungrammatical is that it has not undergone an obligatory contraction.

The existence of obligatory transformations makes transformational theory more abstract, since patterns can be assumed which never turn up explicitly.