Если касса закрыта, я покупаю билет в билетном автомате.

Because касса is a feminine singular noun, and закрыта is the short form of the adjective закрытый (closed).

In Russian, short-form adjectives are often used after to be in sentences like this:

• касса закрыта = the ticket office is closed
• магазин закрыт = the shop is closed
• дверь закрыта = the door is closed

So:

• masculine: закрыт
• feminine: закрыта
• neuter: закрыто
• plural: закрыты

English just says is closed, but Russian changes the form to match the noun.

Покупаю is the imperfective verb form, and here it gives a habitual/general meaning:

• Если касса закрыта, я покупаю билет в билетном автомате. = If the ticket office is closed, I buy / I usually buy a ticket from the machine.

This sounds like a regular action or a general rule.

If you said куплю, that would normally mean a single future completed action:

• Если касса будет закрыта, я куплю билет в автомате. = If the ticket office is closed, I’ll buy a ticket from the machine.

So the difference is mainly:

• покупаю = what I do in general / habitually
• куплю = what I will do in one future situation

It is in the accusative — it just happens to look the same as the nominative.

Билет is:

• masculine
• singular
• inanimate

For masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: билет
• accusative: билет

That’s why after покупаю (I buy), you still see билет.

Compare with an animate noun:

• Я вижу студента = I see a student

There the accusative changes.

Because after в meaning in/at, Russian often uses the prepositional case for location.

The basic form is:

• билетный автомат = ticket machine / ticket vending machine

But after в for location, it becomes:

• в билетном автомате = in/at the ticket machine

Both words change because the adjective has to agree with the noun:

• билетный → билетном
• автомат → автомате

So this is a normal adjective + noun case change.

Yes, literally it looks like in the ticket machine, but idiomatically it means at/from the ticket machine.

Russian often uses location wording where English prefers from or at:

• купить билет в автомате = to buy a ticket from a machine
• платить в кассе = to pay at the ticket office / cashier

So don’t translate it too literally every time. In real usage, в автомате is a normal way to say from the machine.

Here касса means the ticket office, ticket window, or cash desk, depending on context.

It does not usually mean the physical box itself. It refers to the place where tickets or payments are handled.

In transport or cinema contexts, касса often means:

• ticket office
• box office
• ticket counter

So in this sentence, Если касса закрыта... means something like If the ticket office/window is closed...

Because Если касса закрыта is a subordinate clause introduced by если (if), and Russian normally separates such clauses with a comma.

So:

• Если касса закрыта, я покупаю билет...

This is standard punctuation in Russian.

English also often uses a comma when the if-clause comes first:

• If the ticket office is closed, I buy a ticket...

Mostly если means if, but in real life it can sometimes feel close to when(ever) in general statements.

In this sentence:

• Если касса закрыта, я покупаю билет в билетном автомате.

the meaning is something like:

• If the ticket office is closed, I buy a ticket from the machine
• or more naturally in English: When the ticket office is closed, I buy a ticket from the machine

The Russian word is still если, but because the whole sentence describes a usual situation, English may translate it more naturally with when.

Yes. Russian word order is more flexible than English word order.

The sentence as given is neutral and natural:

• Если касса закрыта, я покупаю билет в билетном автомате.

But other versions are also possible, for example:

• Я покупаю билет в билетном автомате, если касса закрыта.
• Если касса закрыта, билет я покупаю в билетном автомате.

The basic meaning stays the same, but the emphasis changes a little.

• putting я покупаю earlier can sound more straightforward
• moving билет can add contrast or emphasis

For learners, the original version is a very good neutral pattern.

Russian has no articles like English a/an and the.

So:

• касса can mean the ticket office or a ticket office
• билет can mean a ticket or the ticket
• билетный автомат can mean a ticket machine or the ticket machine

The exact meaning comes from context.

In this sentence, English naturally uses the ticket office and a ticket, but Russian does not need separate words for that.

Yes, it is a normal and understandable phrase.

It is made from:

• билет = ticket
• билетный = ticket- / for tickets
• автомат = machine / vending machine / automatic machine

So билетный автомат is literally ticket machine.

In everyday use, Russians may also say simply автомат, if the context already makes it clear what kind of machine is meant.

The stress is:

• Е́сли
• ка́сса
• закры́та
• покупа́ю
• биле́т
• в биле́тном автома́те

A rough pronunciation guide:

• Если = YES-li
• касса = KAH-suh
• закрыта = zuh-krih-TAH
• покупаю = puh-koo-PAH-yu
• билет = bee-LYET
• автомате = af-tuh-MAH-tye

Stress is very important in Russian, so it’s good to learn words together with their stress.