Они наняли водителя.

Because водителя is the accusative singular form of водитель.

In this sentence, водитель is the direct object of наняли (hired), so it has to go into the accusative case.

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

• nominative: водитель = driver
• accusative: водителя = driver (as the object)

That is why you see Они наняли водителя, not Они наняли водитель.

The dictionary form is водитель.

That is the nominative singular, the form you would find in a vocabulary list or dictionary. In the sentence, it changes to водителя because of case.

So you can think of it like this:

• dictionary form: водитель
• form used here: водителя

It is singular.

Even though the ending may look unfamiliar, водителя here means one driver.

Compare:

• водителя = driver / a driver / the driver as a singular animate object
• водителей = drivers as a plural animate object

So Они наняли водителя means they hired one driver.

Наняли is the past tense plural form of the verb нанять.

Breaking it down:

• нанять = to hire (perfective)
• наняли = they hired

Russian past tense agrees with the subject in number, and in the singular also in gender. Since они is plural, the verb is in the plural past form: наняли.

In the Russian past tense, gender is only shown in the singular.

Compare:

• он нанял = he hired
• она наняла = she hired
• оно наняло = it hired
• они наняли = they hired

So with они, you only need the plural form, not masculine/feminine/neuter.

Russian has no articles, so there is no direct equivalent of a or the in most sentences.

That means водителя could mean:

• a driver
• the driver

Which one sounds best depends on context. Out of context, English speakers often translate it as They hired a driver, but Russian itself does not force that distinction here.

It can often be translated either way, depending on context.

Because нанять is a perfective verb, the sentence presents the action as completed. In English, that may come out as:

• They hired a driver
• They have hired a driver

In many situations, They hired a driver is the most neutral translation. But if the context emphasizes the present result, English might prefer They have hired a driver.

This is a question of aspect.

• наняли comes from нанять and is perfective
• нанимали comes from нанимать and is imperfective

Наняли focuses on a completed result:

• Они наняли водителя = They hired a driver.

Нанимали would suggest an ongoing, repeated, or process-based action:

• Они нанимали водителя could mean something like They were hiring a driver, They used to hire a driver, or it could depend heavily on context.

So in your sentence, the perfective form tells you the hiring was completed.

Yes. Russian word order is more flexible than English because case endings show who is doing what.

The neutral order here is:

• Они наняли водителя.

But other orders are possible for emphasis, contrast, or style, for example:

• Водителя они наняли.
• Наняли они водителя.

These versions still mean roughly the same thing, but the emphasis changes. The standard, neutral order for a learner is subject + verb + object.

Because они is in the nominative case, which is the normal case for the subject of a sentence.

Meanwhile, водителя is in the accusative form, marking it as the direct object.

So the roles are clear:

• они = the people doing the action
• наняли = hired
• водителя = the person they hired

That is one of the big advantages of Russian case endings: they help show grammatical function even when word order changes.