Подрядная организация выбрала субподрядчика.

Because it is the direct object of the verb выбрала, so it is in the accusative case.

The noun субподрядчик is:

• masculine
• singular
• animate

For animate masculine nouns, the accusative singular looks like the genitive singular. So:

• nominative: субподрядчик
• accusative: субподрядчика

That is why you see субподрядчика, not субподрядчик.

Because подрядная is an adjective modifying организация.

The noun организация is:

• feminine
• singular
• nominative

So the adjective has to agree with it in:

• gender
• number
• case

That gives:

• подрядная организация = feminine singular nominative

If the noun changed, the adjective would change too.

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

The subject here is организация, which is a feminine singular noun, so the past-tense verb is also feminine singular:

• masculine: выбрал
• feminine: выбрала
• neuter: выбрало
• plural: выбрали

Even though an organization is not biologically female, Russian grammar still treats организация as feminine because the noun itself is feminine.

Выбрала is:

• past tense
• perfective aspect

The infinitive is выбрать.

The perfective aspect shows the action as completed: the contractor organization made the choice.

Compare:

• выбрала = chose / selected, with the result completed
• выбирала = was choosing / chose in the process / used to choose, without focusing on completion

So this sentence presents the selection as a finished event.

Yes, sometimes, but the nuance is a little different.

• подрядчик = contractor
• подрядная организация = contracting organization / contractor organization

Подрядная организация sounds more formal and businesslike. It emphasizes that this is an organization/company, not just a person or a general role.

So in legal, technical, or administrative contexts, подрядная организация is very natural.

No, Russian word order is fairly flexible.

The sentence:

• Подрядная организация выбрала субподрядчика.

is the most neutral order:

• subject
• verb
• object

But Russian can change the order for emphasis, contrast, or context. For example:

• Субподрядчика выбрала подрядная организация.
  Emphasis on the subcontractor
• Выбрала подрядная организация субподрядчика.
  Less neutral, more context-driven

So the original order is the safest and most standard one for a plain statement.

Russian has no articles.

So подрядная организация can mean:

• the contractor organization
• a contractor organization

And субподрядчика can mean:

• the subcontractor
• a subcontractor

Which one is intended depends on context. English must choose between a and the, but Russian does not.

Mostly from the case endings and the verb form.

Here:

• Подрядная организация is in the nominative case, so it is the subject
• субподрядчика is in the accusative case, so it is the direct object

So the meaning is clearly:

• the contractor organization did the choosing
• the subcontractor was chosen

This is one reason Russian can allow flexible word order: the endings carry grammatical information.

The main stresses are:

• подря́дная
• организа́ция
• вы́брала
• субподря́дчика

A rough pronunciation guide:

• pad-RYAD-na-ya ar-ga-ni-ZA-tsi-ya VY-bra-la soob-pad-RYAD-chi-ka

A learner does not need perfect English-style transliteration, but the stress matters a lot in Russian, so it is worth learning each word with its stressed syllable.

It can be either, depending on context.

The verb выбрать means to choose / to select. In business or formal contexts, English often translates it more naturally as selected.

So for this sentence:

• chose is correct
• selected may sound more natural in professional English

The Russian grammar is the same either way.

The dictionary forms are:

• подрядный = contracting / contractor-related
• организация = organization
• выбрать = to choose, to select
• субподрядчик = subcontractor

In the sentence, some of these forms change because of grammar:

• подрядный → подрядная to match a feminine noun
• выбрать → выбрала for past tense, feminine singular
• субподрядчик → субподрядчика for accusative singular animate

This is a very common pattern in Russian: dictionary forms change to show the word’s role in the sentence.