Learn Hardcore Russian: Различие между старым мостом и новым мостом интересно. - The difference between the old bridge and the new bridge is interesting.

Различие (между кем? чем?) старым мостом и новым мостом интересно.
(The difference between what? between the old bridge and the new bridge is interesting.)

Using nominative (старый мост, новый мост) after между would be ungrammatical in standard Russian.

How are the instrumental forms старым, новым, and мостом formed?

Here are the forms step by step.

Adjectives (masculine singular)
- старый → stem стар-
  - ending -ым → старым
- новый → stem нов-
  - ending -ым → новым
In general, for masculine/neuter singular adjectives:
- Hard stem: -ый / -ий → -ым / -им in the instrumental
  - красивый → красивым
  - синий → синим
Noun (masculine singular, hard stem)
- мост → add the instrumental ending -ом → мостом
Typical masc. noun patterns:
- Hard consonant: стол → столом, дом → домом
- Soft consonant / -й: герой → героем, учитель → учителем

So старым мостом, новым мостом are just regular instrumental singular forms.

Does между always use the instrumental case?

In modern standard Russian, yes: with normal nouns and pronouns, между is followed by the instrumental.

Examples:

между домом и садом – between the house and the garden
между нами – between us
между этим и тем – between this and that

Very old-fashioned or set expressions with the genitive can exist in literature, but for everyday, correct modern Russian, you should learn:

между + instrumental

Where is the word “is” in this sentence? Why is there no verb like есть?

Russian often omits the verb “to be” in the present tense when it is simply linking a subject and a description.

English:

The difference between the old bridge and the new bridge *is interesting.*

Russian:

Различие между старым мостом и новым мостом интересно.
(literally: The difference between the old bridge and the new bridge interesting.)

In the present tense, you normally do not say есть here:

Различие … интересно. ✅ (normal)
Различие … есть интересно. ❌ (wrong)

In the past or future, a form of быть appears:

Различие … было интересно. – The difference was interesting.
Различие … будет интересно. – The difference will be interesting.

Why is it интересно and not интересное or интересна?

All three can exist in Russian, but they are not interchangeable:

интересно – impersonal, “it is interesting” (a state / evaluation)
- Различие между старым мостом и новым мостом интересно.
  This is felt as a somewhat impersonal evaluation:
  It is interesting (to us) — the difference between the old and new bridge.
Grammatically, интересно here is a predicative form (often grouped with short-form adjectives or as an adverb-like “category of state”). It does not agree in gender with различие, it just states a general fact/state.
интересное – full adjective, agreeing with различие (neuter)
- Различие между старым мостом и новым мостом интересное.
  Here you are describing the difference as having the property “interesting”.
  Roughly: The difference is an interesting one.
интересна – short-form feminine, so would match a feminine noun
- You could say: Эта тема интересна. – This topic is interesting.
- But различие is neuter, so интересна cannot be used with it.

In short:

интересно – “It is interesting (as a fact / to someone)” – most neutral and idiomatic here.
интересное – more descriptive, emphasizes the quality of the difference itself.

Is Различие the subject of the sentence? How can I recognize that?

Yes, Различие is the subject.

You can see this because:

It is in the nominative case:
- Различие – nominative singular (no preposition in front, basic dictionary form).
The structure mirrors English:
- English: The difference between A and B is interesting.
- Russian: Различие между A и B интересно.

Everything after между is in the instrumental, so that whole phrase между старым мостом и новым мостом is a phrase that modifies “различие” (tells you which difference).

So, syntactically:

Subject: Различие между старым мостом и новым мостом
Predicate: интересно

Could I say Различие между старым и новым мостом instead of repeating мостом?

Yes, and that is actually more natural.

Both are correct:

Различие между старым мостом и новым мостом интересно. ✅
Различие между старым и новым мостом интересно. ✅ (more idiomatic)

In the second version:

старым and новым are both adjectives in the instrumental,
мостом (instrumental singular) comes once and is understood to apply to both.

This is a very common pattern in Russian:

между старым и новым домом – between the old and the new house
между русским и английским языком – between Russian and English (language)

Could I change the word order, like Интересно различие между старым мостом и новым мостом?

Yes, that word order is possible, but it slightly changes the focus.

Neutral / most typical:
- Различие между старым (и) новым мостом интересно.
  Focus is on различие (“the difference is interesting”).
Fronting the predicate:
- Интересно различие между старым и новым мостом.
  This puts emphasis on интересно (the evaluation), like: Interesting is the difference between the old and new bridge.

This fronted version sounds more stylistic/literary or like spoken emphasis. It’s not wrong, just less neutral.

For day‑to‑day usage, the original word order is what you should prefer.

What is the difference between различие and разница? Could I say Разница между…?

You can say Разница между старым мостом и новым мостом интересна/интересная/интересная вещь, but there is a nuance:

различие – more formal, more abstract, often used in written language, technical, academic, or careful speech.
разница – more colloquial, everyday word for difference.

So:

Различие между старым и новым мостом интересно. – sounds quite neutral-formal.
Разница между старым и новым мостом интересна. – sounds a bit more everyday, conversational.

Both are grammatically fine; choice depends on style, not on strict grammar.

If the noun were plural (several bridges), how would the forms after между change?

With plural, both adjectives and nouns use instrumental plural.

Example:

Nominative: старые мосты, новые мосты
Instrumental plural:
- старые → старыми
- новые → новыми
- мосты → мостами

So you would say:

Различие между старыми мостами и новыми мостами интересно.
or more naturally:
Различие между старыми и новыми мостами интересно.

Could I express this idea without the noun различие, using a verb instead?

Yes. A common alternative uses the verb отличаться (to differ) or различаться.

For example:

Старый мост и новый мост отличаются. – The old bridge and the new bridge are different.
(You’d usually add чем or по чему: отличаются по высоте, etc.)

Or more explicit:

Старый мост отличается от нового моста. – The old bridge differs from the new bridge.

Your original sentence:

Различие между старым и новым мостом интересно.
emphasizes the difference as a thing (“the difference is interesting”).

Using отличаться/различаться emphasizes the objects themselves and the fact that they are different.

Why is the verb-like word интересно in the neuter form? Shouldn’t it agree with различие somehow?

This touches a subtle point of Russian grammar.

интересно here acts as a predicative form (sometimes treated as a short-form adjective, sometimes as an adverb-like state word).
In this impersonal use, интересно does not change for gender or number. It stays интересно whether the subject is masculine, feminine, neuter, or plural:
- Этот фильм интересен. (short-form adjective, masculine)
- Эта книга интересна. (short-form adjective, feminine)
- Эти книги интересны. (short-form adjective, plural)
But:
- Смотреть этот фильм интересно. – It is interesting to watch this film.
- Различие между старым и новым мостом интересно. – It is interesting, the difference between the old and new bridge.

In structures like your sentence, you can think of интересно as meaning:

It is interesting (that/this fact)…

So it doesn’t need to change to match различие; it behaves more like an impersonal predicate.