Learn Hardcore Russian: Интересно сравнить старый мост и новый мост. - It is interesting to compare the old bridge and the new bridge.

Russian often uses short neuter adjectives / words of state like Интересно, Скучно, Трудно, Легко in an impersonal way, roughly like English “It is interesting / boring / hard / easy”.

“I wonder” is another use of Интересно, usually followed by a clause with ли, что, etc., e.g.:

Интересно, что он скажет. – I wonder what he will say.

That’s a different pattern from your sentence, which is Интересно + infinitive.

Why is there no subject like “it” in the Russian sentence?

Russian often uses impersonal sentences where English would use a dummy subject “it”.

English: It is interesting to compare the old bridge and the new bridge.
Russian: Интересно сравнить старый мост и новый мост.
(literally: “Interesting to compare the old bridge and the new bridge.”)

There is no explicit subject (no “it”) in Russian; the sentence is centered around the state (“interesting”) and the infinitive action (“to compare”).

If you want to show for whom it is interesting, you can add a dative pronoun:

Мне интересно сравнить старый мост и новый мост.
= It is interesting for me to compare the old bridge and the new bridge.
(literally: “To me is interesting to compare...”)

Why is сравнить in the infinitive, and why is it perfective, not сравнивать?

The infinitive сравнить is used because the structure is:

Интересно + infinitive
“(It is) interesting to do X.”

So you need an infinitive: сравнить (“to compare”).

As for aspect:

сравнить – perfective infinitive (one complete act of comparing)
сравнивать – imperfective infinitive (a process / repeated action, “to be comparing, to compare regularly”)

Here, сравнить suggests a single act:

Интересно сравнить старый мост и новый мост.
≈ “It would be interesting (once) to compare the old bridge and the new bridge.”

If you said:

Интересно сравнивать старый мост и новый мост.

this would sound more like:

“It is interesting (in general) to compare the old bridge and the new bridge (as an activity / repeatedly).”

So:

сравнить – more like a specific comparison you might do now or on one occasion.
сравнивать – comparison as an ongoing or habitual process.

Both can be grammatically correct; they just give different aspectual nuances.

Старый мост and новый мост look like nominative singular. But they are objects of сравнить, so shouldn’t they be accusative? Why don’t they change?

They are in the accusative, but for this noun the accusative form = nominative form.

For masculine inanimate nouns like мост:

Nominative singular: мост
Accusative singular: мост (same form)

Adjectives agree with the noun, and for masculine inanimate singular, nominative and accusative are also the same:

старый мост (nom. / acc.)
новый мост (nom. / acc.)

So, even though старый мост and новый мост look like nominative, in this sentence they are accusative objects of сравнить.

If the noun were animate, you would normally see a difference. For example, with мальчик:

Nominative: старый мальчик
Accusative: старого мальчика (animate → same as genitive)

Could I say Интересно сравнить старый и новый мост without repeating мост?

Yes, that is possible and natural:

Интересно сравнить старый и новый мост.

Here, старый and новый both describe мост, and context tells you that you mean the old bridge and the new bridge.

Nuance:

Интересно сравнить старый мост и новый мост.
– very clear and explicit: you are talking about two bridges.
Интересно сравнить старый и новый мост.
– also fine; the repetition is simply avoided for style.

In speech and writing, both versions are used. Repeating мост can sound a bit more emphatic or careful.

Is Интересно сравнить старый мост и новый мост correct, or should it be сравнить старый мост с новым мостом?

Both constructions are correct, but they have slightly different grammar.

With “и” (and) – both direct objects:
- Интересно сравнить старый мост и новый мост.
  → “It is interesting to compare the old bridge and the new bridge.”
Here, старый мост and новый мост are two direct objects of сравнить.
With “с” + instrumental – classical “compare A with B”:
- Интересно сравнить старый мост с новым мостом.
  (старый мост – accusative; с новым мостом – instrumental)
  → “It is interesting to compare the old bridge with the new bridge.”

Both are idiomatic. Russians do often say:

сравнить А и Б
сравнить А с Б

Some speakers find сравнить А с Б slightly more “textbook-style” or explicit, but сравнить А и Б is very common in real speech.

Could I say Мне интересно сравнить старый мост и новый мост? How is that different?

Yes, that is perfectly correct and common:

Мне интересно сравнить старый мост и новый мост.
= “It is interesting for me to compare the old bridge and the new bridge.”

Difference:

Интересно сравнить старый мост и новый мост.
– just states that this action is interesting, without saying to whom.
Мне интересно сравнить...
– specifies that I find it interesting.

You can use other dative pronouns or nouns in the same way:

Тебе интересно сравнить... – It is interesting for you to compare...
Нам интересно сравнить... – It is interesting for us to compare...
Студентам интересно сравнить... – It is interesting for the students to compare...

Why is it Интересно, not Интересное or Интересный?

Here, Интересно is a short neuter form used as a predicative word of state, not an ordinary attributive adjective.

Compare:

Short form / predicative (what it is like):
- Интересно сравнить... – “(It is) interesting to compare...”
- Здесь интересно. – “It is interesting here.”
- Мне скучно. – “I am bored.” / “It is boring to me.”
Long form / attributive (describing a noun):
- интересный фильм – an interesting film
- интересная книга – an interesting book

In Интересно сравнить... there is no noun that интересный/интересная could modify. Instead, the word describes a state: the situation of comparing is interesting. That’s why the short form “Интересно” is used.

Is the word order fixed? Could I say Сравнить старый мост и новый мост интересно?

The natural, neutral word order is:

Интересно сравнить старый мост и новый мост.

You could say Сравнить старый мост и новый мост интересно, but it would:

sound less neutral, a bit stylistically marked or “reversed”,
often be used only in special contexts (e.g. as a contrast: Сравнить их интересно, а понять трудно. – “It’s interesting to compare them, but hard to understand them.”).

For everyday, neutral speech, it’s better to keep:

Интересно + infinitive + objects

How do you pronounce and stress the words in this sentence?

Approximate stresses (stressed syllables in capitals):

ИнтерЕсно – in-te-RYES-na (stress on -ре-)
сравнИть – sra-vNEET' (stress on -нить)
стАрый – STAH-ryy (stress on ста-)
мОст – MOST (stress on мост)
новЫй – na-VYY (stress on -вый)

Whole sentence with stresses:

ИнтерЕсно сравнИть стАрый мОст и новЫй мОст.

Do I need any commas in this sentence?

No, you do not need any commas here.

Интересно сравнить старый мост и новый мост.

Reasons:

Интересно сравнить is a main predicate + infinitive construction.
старый мост и новый мост is just a simple coordination with и.
There is no subordinate clause or anything that needs to be set off by commas.

So the sentence is correctly written without commas.