После акта возврата ячейку для хранения опечатали контрольной пломбой.

Because после always takes the genitive case. The dictionary form is акт, but after после it becomes акта.

So:

• акт = nominative
• после акта = after the act/report/document

Because возврата depends on акт.

Russian often uses a noun + genitive noun structure where English would use of:

• акт возврата = act/report/certificate of return

So in после акта возврата:

• акта is genitive because of после
• возврата is genitive because it belongs to акт

This is a very common Russian pattern.

In official or bureaucratic Russian, акт often means an official document, report, record, or certificate that formally confirms something.

So акт возврата is not an act like a deed or performance. It is more like:

• a return document
• a return report
• a certificate recording the return

The exact English choice depends on context.

Ячейку is the accusative singular of ячейка.

It is in the accusative because it is the direct object of опечатали:

• опечатали что? — ячейку

So:

• ячейка = nominative
• ячейку = accusative

In this context, ячейка usually means a storage compartment, locker, box, or sometimes a safe-deposit box, not literally a biological cell.

Для normally takes the genitive case, so хранение becomes хранения.

The phrase для хранения means for storage or for keeping things.

So:

• ячейка для хранения = a storage compartment / storage box / locker

This is a purpose phrase: it tells you what the compartment is used for.

This is a very common Russian pattern called an indefinite-personal construction.

Опечатали literally looks like they sealed, but Russian often uses this form when the people who did the action are either:

• unknown,
• unimportant,
• obvious from context,
• or deliberately left unnamed.

So the sentence does not have to mention who did it. In English, we often translate this either as:

• they sealed the compartment
• or a passive: the compartment was sealed

In official Russian, this kind of subjectless plural is very natural.

Опечатали is the past plural form of the perfective verb опечатать.

The perfective is used because the sentence describes a completed event:

• first the return document was completed,
• then the compartment was sealed.

The action is presented as a single finished result, not as a process.

If you used an imperfective form such as опечатывали, it would suggest something like:

• an ongoing process,
• repeated action,
• or background information,

which does not fit as well here.

Here опечатали means sealed up, in the sense of closing something officially so that opening it would be evident or prohibited.

It comes from опечатать, which is related to печать in the sense of a seal/stamp.

So in this sentence it has the administrative meaning to seal.

They look similar, but here you should think of печать as seal/stamp, not typo.

• опечатка = a typo / misprint
• опечатать here = to seal officially

Russian has several words built from the same root, so context matters a lot. In this sentence, because of пломбой, the meaning is clearly sealed, not anything connected with a typo.

Because Russian often uses the instrumental case to show the means or instrument used to do something.

Here the question is:

• опечатали чем? — контрольной пломбой

So контрольной пломбой means with a control/security seal.

Both words are instrumental singular feminine:

• контрольная → контрольной
• пломба → пломбой

A контрольная пломба is a seal placed on something to show whether it has been opened, disturbed, or tampered with.

Depending on context, you might think of it as:

• a security seal
• a tamper seal
• an inspection seal
• a control seal

So the idea is not just closing the compartment, but securing it in a way that can be checked later.

Yes, the word order could be different, because Russian word order is fairly flexible.

The sentence begins with После акта возврата to set the time/background frame first: after the return document.

That is very natural in formal or bureaucratic Russian. It tells the reader immediately when the sealing happened.

A different order is possible, for example:

• Ячейку для хранения опечатали контрольной пломбой после акта возврата.

This is still grammatical, but the original order sounds more official and puts stronger emphasis on the sequence of events.