Большие ворота были закрыты.

Because ворота is a plural-only noun in Russian. It refers to a gate as a structure, but grammatically it behaves like a plural noun.

So Russian treats it like gates rather than gate:

• большие ворота = large gate / large gates
• ворота были закрыты = the gate was closed / the gates were closed

That is why both the adjective and the past-tense verb are plural:

• большие = plural form of большой
• были = plural past form of быть

Here ворота is in the nominative plural, because it is the subject of the sentence.

Structure:

• Большие ворота = the subject
• были закрыты = the predicate, were closed

So the sentence literally has the structure:

• Large gates were closed

Even though English may translate it as The large gate was closed, Russian grammar still uses the plural because ворота is grammatically plural.

Because закрыты is the short form of закрытый, and short-form adjectives are commonly used after быть to express a state.

So:

• ворота были закрыты = the gate was closed / the gates were closed
  → describes the condition or state

Compare:

• закрытые ворота = closed gates
  → this is the full form, used before a noun as an attribute

So:

• закрытые ворота = closed gates
• ворота были закрыты = the gates were closed

This is a very common pattern in Russian.

In this sentence, learners often think of it as a short-form passive participle used like an adjective of state.

In practical terms, for understanding the sentence, the important thing is:

• it agrees with ворота
• it means the gates were in the state of being closed

You will often see Russian use forms like this after быть:

• дверь была открыта = the door was open / had been opened
• окна были закрыты = the windows were closed
• ворота были закрыты = the gate was closed

So whether you label it short adjective or short passive participle, the key point for a learner is that it expresses a resulting state very naturally in Russian.

Russian has no articles like a or the.

So Большие ворота были закрыты can mean:

• The large gate was closed
• Large gates were closed

Which one sounds best depends on context. In many real situations, context makes it obvious that we mean the gate.

Russian usually leaves definiteness to context, word order, or the situation rather than using articles.

Grammatically, it literally means big gates, because ворота is plural in form.

But in actual meaning, it often refers to one gate structure—for example, a large entrance gate with two leaves.

So depending on context, English may translate it as:

• big gates
• a big gate
• the large gate

This is one of those places where Russian grammar and English translation do not line up perfectly.

Yes. That would mean:

• The gate was big and closed
• literally, The gates were large and closed

Here:

• большие describes size
• закрыты describes state

You can also compare the two patterns:

• большие ворота = large gates
  (большие directly modifies the noun)
• ворота были большие = the gates were large
  (большие is part of the predicate)

So in your original sentence, большие is an attribute to ворота, while закрыты is the predicative part.

Yes, Russian word order is flexible.

The neutral order here is:

• Большие ворота были закрыты.

But other orders are possible, depending on emphasis:

• Ворота были закрыты. = neutral, no mention of size
• Закрыты были большие ворота. = emphasizes that it was the large gate that was closed
• Были закрыты большие ворота. = also possible, often more literary or context-driven

Russian uses word order partly to show topic and focus, not just grammar.
So the original version is a normal, straightforward sentence.

Sometimes, yes, depending on context.

были закрыты often describes a state in the past:

• The gate was closed

But in some contexts, English might translate it as:

• The gate had been closed

if the focus is on the resulting state after someone closed it.

Russian does not always separate these meanings as sharply as English does. The sentence mainly tells you that, at that past time, the gates were in a closed condition.

Большой is the normal, basic word for big / large in many everyday contexts.

So:

• большие ворота = the natural way to say large gates

Крупный can also mean large, but it is often used for things like:

• scale
• size in a more formal sense
• important or major things

For a physical object like a gate, большие is the most natural choice in a simple sentence.

In standard modern Russian, ворота is generally treated as a plural-only noun (pluralia tantum).

That means you normally do not use a singular form to mean one gate structure.

This is similar to some English nouns that are often plural in form, such as:

• scissors
• pants
• glasses

The meaning may refer to one object, but the grammar is plural.
Russian ворота works in a similar way.