Девочка закрыла калитку и пошла в сад.

Девочка means girl in the sense of a child.
Девушка usually means a young woman or teenage girl.

So if the sentence is about a child, девочка is the natural choice.

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

The infinitive is закрыть (to close). In the past tense:

• закрыл = he closed
• закрыла = she closed
• закрыло = it closed
• закрыли = they closed

Because девочка is feminine singular, the verb is закрыла.

Because калитку is the accusative case.

The noun калитка is the thing being closed, so it is the direct object of закрыла.

For many feminine nouns ending in -а, the accusative singular changes to -у:

• nominative: калитка
• accusative: калитку

So:

• калитка = a gate/wicket gate (subject form)
• закрыла калитку = closed the gate (object form)

Калитка is usually a small gate, especially one in a fence or garden fence.

It is not the usual word for a large gate.
Some related words:

• дверь = door
• ворота = large gate(s)
• калитка = small gate, wicket gate

So in a sentence about going into a garden, калитка is very natural.

This is a very common question.

• идти / шла = to be going, to walk, movement in progress
• пойти / пошла = to start going, to set off

So:

• шла в сад = was going/walked to the garden
• пошла в сад = started going to the garden / went off to the garden

In this sentence, закрыла калитку и пошла в сад presents two completed actions in sequence:

1. she closed the gate
2. then she went into the garden

So пошла fits well because it marks the next action beginning after the first one.

Because в сад expresses motion into the garden, while в саду expresses location in the garden.

Russian often uses:

• в + accusative = into, to
• в + prepositional = in, inside

So:

• пошла в сад = went into the garden
• была в саду = was in the garden

Here there is movement toward/into the garden, so сад is in the accusative.

Because some masculine nouns have the same form in nominative and accusative singular.

For сад:

• nominative: сад
• accusative: сад

So the case changes, but the form does not.

This is very common with inanimate masculine nouns.

Russian has no articles like English a or the.

So девочка can mean:

• a girl
• the girl

And калитку can mean:

• a gate
• the gate

The exact meaning depends on context. Russian speakers understand definiteness from the situation, word choice, and context rather than from articles.

Because Russian does not need to repeat the subject when it is already clear.

So:

• Девочка закрыла калитку и пошла в сад.

naturally means:

• The girl closed the gate and went into the garden.

Russian often omits repeated pronouns and nouns when the subject is obvious. Repeating она here would usually sound unnecessary.

Yes.

• закрыла comes from закрыть — perfective
• пошла comes from пойти — perfective

Perfective verbs often present actions as completed or as single whole events.

That works well here because the sentence tells a sequence of finished events:

• she closed the gate
• she went into the garden

If imperfective verbs were used, the meaning or emphasis would change.

Yes, Russian word order is flexible, although the neutral order here is very natural:

• Девочка закрыла калитку и пошла в сад.

Other orders are possible, for example to change emphasis, but they may sound less neutral.

For example:

• Калитку девочка закрыла и пошла в сад.
  This puts more emphasis on калитку.

In ordinary narration, the original order is the most straightforward.

Пошла is pronounced roughly like pa-SHLA.

It comes from the infinitive пойти, but Russian past-tense forms are built from the past stem, not by keeping the whole infinitive shape.

Compare:

• пойти = to go, to set off
• пошёл = he went / set off
• пошла = she went / set off

So the form may look quite different from the infinitive. This is normal for many Russian verbs.