Штраф решили аннулировать.

Russian often uses a plural verb with no expressed subject to mean someone / people / the authorities / they in a vague, unspecified way.

So решили literally looks like they decided, but it does not mean a specific they has to be named in the sentence. In English, this is often translated more naturally as:

• They decided to cancel the fine
• It was decided to cancel the fine

This is a very common Russian pattern when the speaker does not want to name who did it, or when that is obvious from context.

It is in the accusative singular, because it is the direct object of аннулировать.

However, штраф is a masculine inanimate noun, and for nouns like that, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: штраф
• accusative: штраф

That is why the form does not change.

A useful comparison:

• Я вижу стол → стол is also masculine inanimate, so accusative = nominative
• Я вижу брата → animate masculine, so the accusative changes

Because решить normally takes an infinitive to say what was decided.

Pattern:

• решить + infinitive

So:

• решили аннулировать = decided to annul / decided to cancel

This works just like English decide to do something.

More examples:

• решили уйти = decided to leave
• решили подождать = decided to wait
• решили изменить план = decided to change the plan

Not necessarily.

The sentence says that the decision was made: they decided to cancel the fine. It focuses on the decision, not directly on the completed result.

So the cancellation may be:

• already carried out,
• about to be carried out,
• or simply planned, depending on context.

If you want to say clearly that the fine was cancelled, Russian might use something like:

• Штраф аннулировали.
• Штраф был аннулирован.

Those focus more directly on the completed action/result.

No, not grammatically.

It is an active construction with an unspecified agent:

• решили = they decided

English often translates this kind of Russian sentence with a passive-like phrase such as it was decided, but the Russian grammar itself is not passive here.

Compare:

• Штраф решили аннулировать. = active, unspecified people made the decision
• Штраф был аннулирован. = true passive, the fine was annulled

So the two are related in meaning, but they are not the same structure.

Yes, you absolutely could say Решили аннулировать штраф.

Russian word order is flexible, and moving words changes the focus or topic, not the basic meaning.

• Решили аннулировать штраф = more neutral order
• Штраф решили аннулировать = puts штраф in a prominent position

Starting with штраф can sound like:

• As for the fine, they decided to cancel it
• or it may highlight which thing was decided about

So the version you were given is natural, especially if the fine is already the topic of discussion.

Because Russian has no articles.

So Russian nouns appear without words equivalent to a/an or the, and the listener figures out definiteness from context.

That means штраф could be understood as:

• the fine
• a fine

In many real contexts, it will be understood as the fine, because the fine has probably already been mentioned or is known from the situation.

They are similar, but аннулировать is usually more formal and often sounds official, legal, or bureaucratic.

It is often used for things like:

• fines
• documents
• registrations
• decisions
• permits

So аннулировать штраф sounds like to annul / invalidate a fine in an official sense.

Отменить is more general and often means cancel in a broader everyday sense.

So:

• аннулировать штраф = formal, official
• отменить встречу = cancel a meeting
• отменить урок = cancel a class

In this sentence, аннулировать is a very natural choice.

The stress is:

• штраф
• реши́ли
• аннули́ровать

So the sentence is pronounced approximately as:

shtraf re-SHEE-li an-noo-LEE-ra-vat’

A few notes:

• штраф has only one syllable, so the stress is automatically there
• реши́ли is stressed on -ши́-
• аннули́ровать is stressed on -ли́-
• the final -ть in аннулировать is soft, not a full extra syllable

The dictionary form is решить.

Here is what you have in the sentence:

• решить = dictionary form, to decide
• решили = past tense, plural

So решили means decided.

Because it is plural, it matches the idea of an unspecified they:

• Они решили = they decided
• Решили = decided / they decided / it was decided

This is one of the first things to check when reading Russian verbs: find the dictionary form, then identify the tense and number.