When should I use the definite article before a plural noun followed by a relative clause?

Question

Here are examples from my posts on Lang-8:

E1. Its style was older -- for example, "today" was written there as "to-day" -- but it had no typos, or at least far fewer of them. Out of interest, I compared both newer and older books with an online program, and there turned out not to be much difference between the words and phrases used in them.

E2. Researchers from the University of Georgia have studied the effect of these innovations on the marks of American pupils studying at elementary school in reading and math.

These are corrected sentences; the articles weren't changed.

E3. That is why the biologist Adam Boyko working in the USA analyzed the DNA of dogs living in the most remote and isolated locations.

Here, I originally wrote the dogs, and it was corrected to dogs.

In all of the three sentences, the words, pupils, and dogs hadn't been mentioned before.

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I found a question on ELU associated with the topic. Here are some excerpts from the accepted answer:

'The' isn't used with plurals when that plural implies a general reference, only when that plural implies a specific group.

The students learn better when the teachers are passionate about teaching.

implies that you're not talking about students in general but a particular group, for example those who study in a certain school or at a certain level; and the same goes for the teachers: you don't mean teachers in general.
_{Source: Omitting article “the” in front of plural nouns? -- ELU}

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My questions:

Q1. If I removed the from the first sentence, would it still be grammatical, and, if so, how would the meaning change?

Q2. Why is it correct not to use the definite article in the second sentence? As it is written in the answer I copied, we are talking about those who study at a certain level.

Q3. What is the difference between dogs and the dogs in the third sentence, especially in comparison with the first one?

Answer 1

My answers:

Q1. If I removed the from the first sentence, would it still be grammatical, and, if so, how would the meaning change?

A1. It would still be grammatical, but the meaning would change.

"...there turned out not to be much difference between the words and phrases used in them"

has the connotation that you want: you're comparing definite sets of words and phrases, namely the ones in the newer versions of the books and the ones in the older versions.

By contrast,

"...there turned out not to be much difference between words and phrases used in them"

might lead a reader to first think that you're comparing words to phrases, not the words and phrases in one version against the words and phrases in another. This is a kind of ambiguity that a reader would probably be able to resolve after a few seconds of thought, but it's better to put him or her through that confusion in the first place.

Q2. Why is it correct not to use the definite article in the second sentence? As it is written in the answer I copied, we are talking about those who study at a certain level.

A2. If instead of "the marks of American pupils studying at elementary school in reading and math"

you had said "the marks of the American pupils studying at elementary school in reading and math"

it would suggest that you're talking about every American pupil in elementary school. But you're only talking about some of them, namely the ones in the sample.

Q3. What is the difference between "dogs" and "the dogs" in the third sentence, especially in comparison with the first one?

A3. Again, Boyko only studied the DNA of some dogs. Using "the dogs" would have suggested that he studied all of them.