Вечером мы закрыли ворота и калитку, а потом ещё раз посмотрели на сквер.

Вечером is the instrumental singular of вечер (evening), but in this kind of expression it works like an adverb meaning in the evening / during the evening.

Russian often uses the instrumental case for time expressions like this:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So Вечером here sets the time for the whole sentence: In the evening, we...

Because the subject is мы (we).

In Russian past tense, the verb agrees with the subject in gender and number:

• я закрыл / закрыла = I closed
• он закрыл = he closed
• она закрыла = she closed
• мы закрыли = we closed
• они закрыли = they closed

So:

• мы закрыли = we closed
• мы посмотрели = we looked

The ending -ли marks plural past.

They are perfective because the sentence describes completed actions in sequence:

1. we closed the gate and the wicket gate
2. then we looked at the square/park once more

Perfective verbs are very common when narrating finished events.

Here:

• закрыли = closed completely
• посмотрели = had a look / looked

If imperfective were used, it could suggest an ongoing process, repetition, or background action instead:

• закрывали = were closing / used to close
• смотрели = were looking / looked repeatedly / used to look

In this sentence, the speaker is reporting a completed chain of events, so perfective is the natural choice.

Not necessarily. Ворота is one of those Russian nouns that are normally used only in the plural. It usually refers to a gate as a structure with two leaves or panels, so grammatically it behaves like a plural noun even when it means one gate.

So:

• ворота = gate
• закрыли ворота = closed the gate

This is similar to how some English words are plural in form but refer to one object, like scissors or pants.

They are two different kinds of entrance:

• ворота = a large gate, usually for vehicles or a wide entrance
• калитка = a small gate, usually for people on foot, often built into or next to a fence

So закрыли ворота и калитку means they closed both the big gate and the smaller pedestrian gate.

Because the verb смотреть / посмотреть usually takes на + accusative when it means to look at something.

So:

• смотреть на дом = to look at the house
• посмотреть на сквер = to look at the square/park

You usually cannot say посмотреть сквер with this meaning.

If you want a verb closer to inspect / examine / look over, Russian may use a different verb, such as осмотреть.

Ещё раз means again, once more, or one more time.

So:

• а потом ещё раз посмотрели на сквер = and then looked at the square once more

It emphasizes repetition: they had already looked before, and then did it again.

A few similar examples:

• Скажи ещё раз. = Say it again.
• Прочитай ещё раз. = Read it one more time.

Потом means then / afterwards.
Adding а gives a natural connection between two parts of the sentence and often sounds like and then / and after that / whereas then, depending on context.

Here а потом helps move the narrative forward:

• Вечером мы закрыли ворота и калитку, а потом...
• In the evening we closed the gate and the wicket gate, and then...

This а does not necessarily mean a strong contrast here. It often just helps connect one event to the next in a smooth, natural way.

Because а is a coordinating conjunction joining two clauses:

• мы закрыли ворота и калитку
• (мы) потом ещё раз посмотрели на сквер

In Russian, a comma is normally used before а when it connects clauses like this.

So the comma marks the boundary between:

1. we closed the gate and the wicket gate
2. and then we looked at the square again

Russian word order is flexible, and placing Вечером first is a very natural way to set the scene. It gives the time frame before the actions.

So the sentence begins with the background information:

• Вечером = In the evening

Then it tells what happened.

Russian often puts time or place expressions first when they provide the setting:

• Утром мы ушли. = In the morning we left.
• На улице было холодно. = Outside it was cold.

It is possible to place вечером elsewhere, but sentence-initial position sounds very natural here.

They are all in the accusative case, because they are objects of the verbs.

• закрыли ворота
  ворота is accusative plural; for this noun, the form looks like the nominative plural

• закрыли калитку
  калитку is accusative singular of калитка

• посмотрели на сквер
  сквер is accusative singular after на with смотреть / посмотреть на

So even though the forms are different, all three are functioning as direct or governed objects in the sentence.