Они в основном работают здесь.

A word-by-word breakdown is:

• Они = they
• в основном = mostly, mainly, for the most part
• работают = work / are working
• здесь = here

So the whole sentence means They mostly work here or They mainly work here.

Russian present tense can often cover both:

• a general/habitual meaning: they work
• a right now / current activity meaning: they are working

The context decides which one is more natural.

In this sentence, в основном usually suggests a general pattern or usual situation, so They mostly work here is the most natural interpretation.

Because it is the 3rd person plural form of работать (to work).

The conjugation is:

• я работаю = I work
• ты работаешь = you work
• он / она работает = he / she works
• мы работаем = we work
• вы работаете = you work
• они работают = they work

So они работают means they work.

В основном is a very common expression meaning:

• mostly
• mainly
• for the most part
• sometimes basically, depending on context

In this sentence, it means that here is the place where they work most of the time or primarily.

So:

• Они работают здесь = They work here.
• Они в основном работают здесь = They mostly/mainly work here.

This is a fixed expression.

Grammatically, основном is in the prepositional case after в. Historically, it comes from something like in the main / in the principal thing, but for learners it is best to treat в основном as a set phrase meaning mostly or mainly.

So you usually do not need to analyze it literally every time. Just learn:

• в основном = mostly / mainly

Yes. Russian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• Они в основном работают здесь.
  Neutral: They mostly work here.

• Они работают здесь в основном.
  Slightly more emphasis on mostly at the end.

• Здесь они в основном работают.
  More emphasis on here.

The version you were given is very natural and neutral.

You can sometimes leave it out if the subject is already clear from context.

For example:

• В основном работают здесь.
  = They mostly work here / Mostly they work here, if the listener already knows who they are.

But Russian uses subject pronouns more often than people sometimes expect, especially when you want to be clear or explicit. In a standalone sentence, они is perfectly natural.

Yes, здесь and тут both mean here.

In many situations they are interchangeable:

• Они в основном работают здесь.
• Они в основном работают тут.

Both are natural.

Very roughly:

• здесь can sound a little more neutral or standard
• тут can sound a little more conversational

But in everyday speech, the difference is often small.

No. Они can refer to:

• people
• animals
• objects
• organizations
• anything plural

So depending on context, они could mean they in any of those senses. In this sentence, it most naturally refers to people, but grammatically it does not have to.

Работать is an imperfective verb.

That matters because imperfective verbs are used for:

• ongoing actions
• repeated actions
• habitual actions
• general facts

This fits the sentence well, because в основном suggests a repeated or usual situation: they generally work here.

The perfective partner is поработать in some contexts, but that does not simply mean to work in the same neutral way. For this sentence, работать is the correct choice.

The stress is:

• онИ
• в основнОм
• рабОтают
• здесь

So you can say it roughly as:

• an-EE v as-nav-NOM ra-BO-ta-yut zdes'

A more natural rhythm is:

• Они́ в основно́м рабо́тают здесь.

Russian does not have articles like a, an, or the.

So Russian simply says:

• Они в основном работают здесь.

And English adds articles only if needed by the context. In this sentence, English does not need one either, so the translation is straightforward.

Usually здесь just means here, so the basic meaning is They mainly work here.

If you specifically want from here, Russian often makes that clearer through context or a different structure. But in some real-life contexts, especially when talking about a base of operations, English might translate it as from here if that sounds more natural.

Still, the most direct reading is simply:

• They mostly work here.