Туча закрыла солнце.

Because туча is in the nominative case, which is the normal case for the subject.

In Туча закрыла солнце:

• туча = cloud / storm cloud → subject
• закрыла = covered / blocked
• солнце = sun → direct object

So the sentence structure is basically:

• Туча = the thing doing the action
• закрыла = the action
• солнце = the thing affected by the action

In the past tense, Russian verbs agree with the subject in gender and number.

The noun туча is:

• feminine
• singular

So the past-tense verb must also be feminine singular:

• закрыл = masculine
• закрыла = feminine
• закрыло = neuter
• закрыли = plural

That is why the sentence has закрыла.

Солнце is in the accusative case, because it is the direct object of the verb закрыла.

However, for many inanimate neuter nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: солнце
• accusative: солнце

That is why it looks unchanged.

Not exactly in the English sense of close a door or a window.

The verb закрыть can mean:

• to close
• to cover
• to block
• to shut off from view

In this sentence, Туча закрыла солнце means something like:

• The cloud covered the sun
• The cloud blocked the sun
• The cloud hid the sun from view

So yes, it is the same Russian verb, but the natural English translation here is usually covered or blocked, not closed.

This is a very common question.

• облако = cloud in a general, neutral sense
• туча = a dark, heavy cloud, often a storm cloud

So туча suggests something large and gloomy that could block the sun. That makes it especially natural in this sentence.

Compare:

• Облако закрыло солнце. = A cloud covered the sun.
• Туча закрыла солнце. = A dark cloud / storm cloud covered the sun.

Russian has no articles.

So туча can mean:

• a cloud
• the cloud

And солнце can mean:

• the sun
• sometimes just sun in a general sense

The exact meaning depends on context.

So Туча закрыла солнце could be translated as:

• A cloud covered the sun
• The cloud covered the sun

Both are possible unless the wider context makes one clearly better.

Yes, Russian word order is more flexible than English.

The neutral order here is:

• Туча закрыла солнце.

But you could also say:

• Солнце закрыла туча.
• Туча солнце закрыла.

The basic meaning stays the same because the cases and verb form show who does what. But the emphasis changes:

• Туча закрыла солнце. = neutral statement
• Солнце закрыла туча. = emphasis on the sun
• Туча солнце закрыла. = can sound more expressive or contrastive depending on context

Закрыла comes from the perfective verb закрыть.

Perfective verbs usually present an action as:

• completed
• single
• result-focused

So Туча закрыла солнце means the cloud ended up covering the sun.

The imperfective partner is закрывать. A past form like закрывала would sound more like:

• it was covering it
• it kept covering it
• it used to cover it
• the action is viewed as ongoing or repeated

So in this sentence, закрыла is natural because it describes a completed event: the sun became covered.

The stress is:

• тУча
• закрылА
• сОлнце

So the full sentence is pronounced approximately:

тУча закрылА сОлнце

A few notes:

• туча sounds roughly like TOO-cha
• закрыла has stress on the last -ла
• солнце has stress on the first syllable, and the л is not strongly pronounced as a full separate sound in careful learner-style pronunciation; native pronunciation compresses the consonant cluster

Yes, that can work in many contexts, especially if you are translating for natural English.

Still, the most direct sense of закрыла here is:

• covered
• blocked
• obscured

So:

• The cloud covered the sun = very direct
• The cloud blocked the sun = also natural
• The cloud hid the sun = possible, but slightly freer

If you want to stay close to the Russian verb, covered is often the best choice.

Because закрыть normally takes a direct object in the accusative case.

So:

• закрыть что? → солнце

That gives:

• закрыла солнце

The genitive would appear in other constructions, but not in this basic direct-object pattern. Here the grammar is straightforward: subject + verb + accusative object.