Длина паромной переправы — одна морская миля.

Because длина means length, and in Russian a noun like length is often followed by the thing whose length it is in the genitive.

• длина чего? = the length of what?
• паромной переправы = of the ferry crossing

The base form is паромная переправа. In the sentence, it changes to the genitive singular:

• паромная переправа → nominative
• паромной переправы → genitive

So the structure is literally:

• длина паромной переправы = the length of the ferry crossing

The dictionary form is паромная переправа.

That is a feminine noun phrase:

• паромная = ferry-related
• переправа = crossing, passage, ferry crossing

In the sentence, both words change to the genitive singular:

• паромная → паромной
• переправа → переправы

This is normal adjective + noun agreement.

In Russian, the verb to be is usually omitted in the present tense.

So instead of saying something like The length of the ferry crossing is one nautical mile, Russian often just puts the two parts next to each other, separated by a dash:

• Длина паромной переправы — одна морская миля.

This dash is very common in written Russian when one noun phrase is being identified as another.

Compare:

• Москва — столица России. = Moscow is the capital of Russia.
• Мой брат — врач. = My brother is a doctor.

In everyday speech, the pause may be more important than the punctuation itself.

Because миля is a feminine noun.

Russian numerals like one agree in gender with the noun:

• один for masculine
• одна for feminine
• одно for neuter

Since миля is feminine, you get:

• одна миля = one mile

Also, the adjective must agree too:

• одна морская миля

It is in the nominative case.

This sentence is an equational sentence:

• Длина паромной переправы = subject/topic
• одна морская миля = predicate noun phrase

In modern Russian, after this kind of dash construction, the second noun phrase is commonly nominative:

• Это проблема.
• Мой брат — учитель.
• Длина переправы — одна миля.

So одна морская миля stays in the nominative.

Переправа means a crossing or means of crossing, especially across a river, strait, or other stretch of water.

Depending on context, it can refer to:

• the crossing route itself
• the transport connection
• the ferry service
• sometimes even the crossing point

So паромная переправа is not just a ferry as a vehicle. It is more like:

• a ferry crossing
• a ferry route
• a ferry connection

That is why this sentence uses переправа, not simply паром.

Паром usually means the ferry boat itself.

Паромная переправа means the ferry crossing/service/route.

So compare:

• Паром большой. = The ferry is big.
• Паромная переправа закрыта. = The ferry crossing is closed.

In your sentence, the length refers to the crossing, not the boat, so паромная переправа is the right choice.

Because Russian is being specific: it means a nautical mile, not some other kind of mile.

• миля = mile
• морская миля = nautical mile

The adjective морская literally means marine/sea-related, so the phrase is:

• морская миля = sea mile = nautical mile

This is a fixed term in Russian.

They show agreement with the noun in gender, number, and case.

1. паромной переправы

The noun переправа is:

• feminine
• singular
• genitive

So the adjective must match:

• паромная переправа → nominative singular feminine
• паромной переправы → genitive singular feminine

2. морская миля

The noun миля is:

• feminine
• singular
• nominative

So the adjective matches that too:

• морская миля

This is one of the most important patterns in Russian: adjectives change to match the noun.

Yes, but the original order is the most neutral for a formal statement.

Neutral order:

• Длина паромной переправы — одна морская миля.

Other orders are possible for emphasis, for example:

• Одна морская миля — длина паромной переправы.

That sounds more marked or stylistic, and less natural in ordinary explanation.

Russian word order is flexible, but not random. The original version is what you would most likely see in a textbook, description, or factual statement.

Yes, absolutely. That is a very natural alternative.

• Длина паромной переправы — одна морская миля.
• Длина паромной переправы составляет одну морскую милю.

The second version is a bit more explicit and often sounds more formal or technical.

Notice the case change:

• with the dash: одна морская миля = nominative
• with составляет: одну морскую милю = accusative

That is because составлять is a verb, and its object goes into the accusative.

It sounds natural, but somewhat informational or descriptive rather than conversational.

You might see it in:

• a guidebook
• a technical description
• a map caption
• a tourist text
• an informational sign

In casual speech, someone might say something simpler depending on context, but this sentence itself is perfectly normal Russian.

A helpful stress guide is:

• длинá
• парóмной
• перепрáвы
• однá
• морскáя
• миля

So roughly:

длинá парóмной перепрáвы — однá морскáя миля

For an English speaker, the important thing is to notice that Russian stress is mobile and must often be learned word by word.

Russian has no articles.

So where English says:

• the length
• a nautical mile

Russian simply says:

• длина
• одна морская миля

Whether something is the, a, or just general is understood from context, word order, and meaning.

That is why a Russian sentence can feel very compact compared to English.