“I don't know what [is] the difference [is] between some words [is]”

Question

I was proofreading an earlier question which was phrased in this way:

i am learning English and i do not know what is different between some words.

Apart from the correction of some other minor details, when I was proofreading the sentence above, I had some doubts while trying to figure out where "is" should be put:

1. I am learning English and I do not know what is the difference between some words.

2. I am learning English and I do not know what the difference is between some words.

3. I am learning English and I do not know what the difference between some words is.

Can anybody explain?

Answer 1

You can dismiss 1 out of hand: this is the structure for a question, where what is used as an interrogative:

I do not know: what is the difference between some words?

Both 2 and 3 are acceptable. What here is the head of a Wh- cleft (pseudo-cleft). Here the basic proposition is The difference is X; what substitutes for X and moves to the front, giving the fixed order what the difference is.

Between some words is a prepositional phrase which may be analyzed either as an adjunct modifying difference or as a complement licensed by difference; for the present purposes it doesn't matter.

When it comes to where we place the prepositional phrase, three principles operate:

1. We try to keep the principal constituents (what, the difference, is) as close together as possible.
2. We try to put 'heavier' phrases at the end.
3. We try to keep adjuncts or complements as close as possible to the constituents they modify or complete.

In this case there is some conflict between them: Principles 1 and 2 both call for the prepositional phrase to come last; but Principle 3 calls for the prepositional phrase to come immediately after difference.

Accordingly, you are free to put it in either place.