Компания решила нанять помощника.

Because the subject компания is a feminine singular noun.

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

• masculine: решил
• feminine: решила
• neuter: решило
• plural: решили

So:

• компания решила = the company decided

Even though company in English is not usually thought of as feminine, in Russian the noun компания is grammatically feminine, so the verb must match it.

Because помощник is the direct object, and it refers to a person. In Russian, masculine animate nouns change in the accusative case.

• nominative: помощник
• accusative: помощника

This happens because Russian treats masculine animate nouns differently from inanimate ones.

Compare:

• Я вижу помощника = I see an assistant
• Я вижу стол = I see a table

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

Компания is in the nominative case because it is the subject of the sentence — the thing doing the action.

The company is the one that decided, so it stays in nominative:

• компания = nominative singular

A very common pattern in Russian is:

• subject in nominative
• object in accusative

So here:

• Компания = subject, nominative
• помощника = object, accusative

Because it depends on решила.

In Russian, after verbs like решить meaning to decide, you usually use an infinitive to say what someone decided to do:

• решить сделать
• решить купить
• решить нанять

So:

• Компания решила нанять помощника
  = The company decided to hire an assistant

This is very similar to English decided to hire.

This is a question of verb aspect.

• нанять = perfective
• нанимать = imperfective

Here, нанять is used because the sentence presents the hiring as a single complete intended action: the company made a decision to hire someone.

Very roughly:

• решила нанять = decided to hire, as a complete step
• решила нанимать = decided to be hiring / to hire in a more process-oriented, repeated, or less bounded sense

In most neutral cases like this one, решила нанять is the natural choice.

It can mean either one, depending on context.

Russian has no articles like a or the, so the noun by itself does not tell you whether it is definite or indefinite.

So помощника could mean:

• an assistant
• the assistant

In an isolated sentence, English often translates it as an assistant, because that sounds most natural without extra context.

No, Russian word order is more flexible than English word order.

The neutral order here is:

• Компания решила нанять помощника.

This sounds natural and straightforward.

But other orders are possible if you want different emphasis, for example:

• Помощника решила нанять компания.
  Emphasizes that it was an assistant that the company decided to hire.

• Нанять помощника решила компания.
  Emphasizes the action of hiring an assistant.

Even though word order can change, the cases help show who is doing what.

Because помощника here is the accusative singular of помощник.

Its basic form is:

• помощник = assistant

The accusative singular for a masculine animate noun is:

• помощника

The plural would be different:

• nominative plural: помощники
• accusative plural: помощников

So помощника here clearly means one assistant.

The stress is:

• компа́ния
• реши́ла
• наня́ть
• помо́щника

So the full sentence is pronounced:

• Компа́ния реши́ла наня́ть помо́щника.

A few useful notes:

• я in наня́ть sounds like ya
• the soft sign ь in наня́ть does not make a sound by itself; it softens the preceding consonant
• щ in помо́щника is pronounced like a soft long sh sound

Usually yes, but remember that компания can also mean group of people or social company in some contexts.

For example:

• крупная компания = a large company
• весёлая компания = a fun group of people

In this sentence, because of решила нанять помощника, the meaning is clearly company in the business sense.

The dictionary forms are:

• компания = company
• решить = to decide
• нанять = to hire
• помощник = assistant, helper

In the sentence:

• компания stays in its basic singular nominative form
• решила is the feminine past tense form of решить
• нанять is already the infinitive
• помощника is the accusative singular form of помощник