Regarding comparative form: the structure after 'than'

Question

I found the following exercise in "Grammar Practice" (written by Penny Ur, p.122, 2016):

Write sentences that express your own opinion, using the comparative form of the adjectives given.
1. Biology / literature (interesting, boring) (abbrev.)

If a learner writes "Biology is more interesting than literature is more interesting" as the answer, is this okay from grammatical and semantic perspectives? I think this answer is understandable from these two points. However, that answer might feel strange to some teachers or students because the subjects ('biology' & 'literature') share the same complement ('more interesting').

Answer 1

No, "Biology is more interesting than literature is more interesting" is not grammatical.* Repeating the word "more" is not right.

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The basic pattern

This sentence illustrates the basic pattern, though it's not how we usually say it:

Light travels faster than sound travels.

There is only one comparison: "faster than". There are two things being compared: how fast light travels and how fast sound travels. We nearly always elide the second verb except in the rare situation where the verbs are different. So, this is the normal way to form the sentence:

Light travels faster than sound.

Here are some examples with "more":

Light travels more quickly than sound.

Iron is more useful than gold.

Gold is worth more than iron.

The entire phrase starting with "more", including the subordinate clause starting with "than", functions as an adverb (as on "travels") or complement.

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The correct answer

So, a correct answer would be:

Biology is more interesting than literature.

The complement of "biology" is "more interesting than literature", not "more interesting".

You could also say either of these, which include one or both of the elided words, but these are unusual; normally we would say them only to make some unusual emphasis:

Biology is more interesting than literature is.

Biology is more interesting than literature is interesting.

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A few more examples

Here's an example of the full sentence pattern, with two different verbs and two objects:

Water flows downhill faster than salmon swim upstream.

And here are a couple examples with two linking verbs:

The clown looks happier than he feels.

The doorman is more useful than he looks.

Most common, though, is the simplest pattern, illustrated by these well known sentences:

That dog's bark is worse than his bite.

Blood is thicker than water.  (A traditional proverb.)

A good name is worth more than silver or gold.  (A Biblical proverb.)

Imagination is more important than knowledge. —Albert Einstein

The pen is mightier than the sword. —Edward Bulwer-Lytton

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* _{Technically, it could be grammatical if set up in a very unusual context, but that's too strange to bother with when mastering the basics.}

Answer 2

Biology is more interesting than [literature is more interesting].

It's understandable, but completely ungrammatical. The bracketed element is a comparative clause serving as complement of "than". Such clauses are obligatorily reduced in some way. Here, the comparative clause has been reduced to just the single element "literature", though the verb "is" could be optionally retained and just the adjective phrase "more interesting" dropped.

The meaning can be given as "Biology is x interesting; literature is y interesting; x > y"

Syntactic structure: "than" is a preposition, and the term 'comparative clause' applies to the subordinate clause expressing the second term of the comparison, as bracketed, not to the matrix clause that expresses the comparison as a whole.

The adjective phrase "more interesting than literature" is predicative complement of "is".