Контрольную пломбу поставят не позднее чем сегодня вечером.

Because it is the direct object of поставят.

• пломба is a feminine noun
• its nominative singular form is пломба
• the accusative singular form is пломбу

The adjective контрольная has to agree with the noun, so it also changes:

• nominative: контрольная пломба
• accusative: контрольную пломбу

So the sentence uses контрольную пломбу because it means (they will install) the control seal as the object of the verb.

Пломба here means a seal, usually an official or tamper-evident one.

In real-life Russian, this often refers to a seal placed on something like:

• a meter
• a container
• a device
• equipment that should not be opened or altered without authorization

So this is not a tooth filling here, even though пломба can also mean a dental filling in other contexts. In this sentence, the meaning is clearly seal.

Yes, literally поставить often means to put or to place, but in Russian it is also used very naturally in a broader sense like:

• install
• attach
• put in place
• set

So контрольную пломбу поставят means something like:

• they will install the seal
• they will place the seal
• they will put on the seal

This is a very normal Russian use of поставить.

Because поставят is the perfective future, and that fits the meaning better here.

• ставить = imperfective
• поставить = perfective
• поставят = they will put/install with focus on the completed result

The sentence is about a deadline: the seal will be installed by a certain time. Russian usually prefers the perfective here because the result matters: the seal will be in place no later than this evening.

If you said будут ставить, it would sound more like focusing on the process, not the completed outcome.

This is a very common Russian pattern: 3rd person plural with no explicit subject can mean someone / they / the relevant people.

So поставят here does not necessarily mean a specific they already mentioned. It often means:

• they in the general sense
• someone will
• the workers / officials / technicians will

English sometimes does something similar with they, but Russian uses this structure very naturally when the doer is unknown, unimportant, or obvious from context.

Не позднее чем means no later than.

So:

• не позднее чем сегодня вечером = no later than this evening

It sets a latest possible time for the action.

A useful way to understand it:

• позднее = later
• не позднее = not later
• чем = than

Together, it forms the standard expression не позднее чем ... = no later than ...

Because чем is used in comparisons, and не позднее чем is built like a comparison structure.

Compare:

• позднее, чем завтра = later than tomorrow
• не позднее чем завтра = no later than tomorrow

Even though in English no later than feels like a fixed phrase, in Russian it still contains the comparative element чем.

Сегодня вечером is the normal, idiomatic way to say this evening / tonight in Russian.

It is a set time expression:

• сегодня утром = this morning
• сегодня днём = this afternoon / during the day
• сегодня вечером = this evening
• сегодня ночью = tonight / during the night

Using сегодняшним вечером is possible in some contexts, but it is much less neutral here and sounds more marked or stylistically different. For ordinary scheduling, сегодня вечером is the standard choice.

Because контрольный often has a broader practical meaning in Russian than a literal word-for-word English equivalent suggests.

In different contexts, контрольный can mean things like:

• control
• inspection
• checking
• official verification
• test / test-related

So контрольная пломба is not a seal that is itself controlling something. It is a seal used for control/inspection purposes, often meaning an official seal showing that something has been checked or should not be tampered with.

The word order is fairly neutral and natural.

• Контрольную пломбу поставят не позднее чем сегодня вечером.

This structure presents:

1. what will be installed
2. the action
3. the deadline

Russian word order is flexible, so other orders are possible, for example:

• Не позднее чем сегодня вечером поставят контрольную пломбу.
• Сегодня вечером, не позднее, поставят контрольную пломбу.

But the original sentence sounds like a normal, straightforward statement. It is especially natural in formal, administrative, or service-related contexts.

Yes. Both are possible.

• не позднее чем = no later than
• не позже чем = no later than

In many situations they mean the same thing. However, не позднее чем often sounds a bit more formal or official, which fits a sentence like this one.

So in an administrative or service notice, не позднее чем is very natural.

Yes, it sounds fairly formal or official.

Reasons:

• контрольная пломба is technical/administrative vocabulary
• не позднее чем is a formal-style time expression
• the unnamed поставят sounds like a notice about what service staff or officials will do

This kind of sentence would fit well in:

• a service notice
• a utility company message
• an official instruction
• a written update from technicians or inspectors