В конюшне стоят две лошади, и фермер приносит им овёс.

Because Russian uses different cases after в depending on whether you mean location or movement.

• в + Prepositional = in / inside a place
• в + Accusative = into a place

Here the horses are already inside the stable, so Russian uses the Prepositional: в конюшне.

If someone were going into the stable, you would say в конюшню.

Russian often uses posture verbs to describe where people, animals, or things are:

• стоять = to stand
• сидеть = to sit
• лежать = to lie

Since horses are typically on their feet, стоят is very natural here.

You could say В конюшне есть две лошади if you only want to say that two horses exist / are present in the stable. But стоят gives a more concrete picture: the horses are there, standing.

Because Russian verbs agree with the subject, not with word order.

The subject is две лошади, which refers to two horses, so the verb is plural: стоят.

Even if the subject comes after the verb, agreement still stays the same:

• В конюшне стоят две лошади
• Две лошади стоят в конюшне

Both use the plural verb.

Because лошадь is a feminine noun.

Russian 2 has two forms:

• два for masculine and neuter nouns
• две for feminine nouns

So:

• два стола = two tables
• два окна = two windows
• две лошади = two horses

After два / две, три, четыре, Russian nouns usually use the special counting form, traditionally described as genitive singular.

So you get:

• одна лошадь
• две лошади
• три лошади
• четыре лошади
• пять лошадей

That is why the sentence has две лошади.

A useful thing to notice: even though the noun has this special form after две, the whole phrase still means more than one animal, so the verb is plural: стоят.

Because приносить usually works like to bring someone something.

Russian asks:

• приносить кому? что?
• to bring to whom? what?

So:

• им is dative = to them
• овёс is the thing being brought

Compare:

• Я вижу их = I see them
• Я приношу им овёс = I bring them oats

So их would be wrong here, because the horses are the recipient, not the direct object.

Because Russian treats овёс as a singular mass noun, similar to how English says rice or wheat.

So although English uses the plural-looking word oats, Russian uses singular:

• овёс

This is very normal. Different languages divide up food and materials differently.

Here овёс is the direct object of приносит, so it is in the accusative case.

But овёс is:

• masculine
• singular
• inanimate

For many masculine inanimate nouns, the accusative looks the same as the nominative.

So:

• Nominative: овёс
• Accusative: овёс

That is why the form does not visibly change.

It can mean either, depending on context.

приносит is:

• present tense
• imperfective

So it can describe:

• a habitual action: the farmer brings them oats
• an action happening in the scene: the farmer is bringing them oats

Russian present-tense imperfective often covers both meanings, and the wider context tells you which one is intended.

The imperfective приносить is used when the action is viewed as:

• ongoing
• repeated
• habitual
• part of a general scene

That fits this sentence well. It sounds like a description of what is happening, or what the farmer regularly does.

The perfective partner is принести. That would be used for a completed bringing action, for example in the past or future:

• фермер принёс им овёс = the farmer brought them oats
• фермер принесёт им овёс = the farmer will bring them oats

Yes. Russian word order is fairly flexible, and changing it usually changes focus rather than basic meaning.

For example:

• В конюшне стоят две лошади = neutral scene-setting
• Две лошади стоят в конюшне = more focus on the horses
• Фермер приносит им овёс = neutral
• Им фермер приносит овёс = more focus on to them

So the original sentence sounds natural and neutral, but other orders are possible.

Because the sentence contains two separate clauses, each with its own verb:

• В конюшне стоят две лошади
• фермер приносит им овёс

When Russian joins two independent clauses with и, it normally uses a comma.

So the comma is there because:

• first clause: стоят
• second clause: приносит

They are connected, but they are still two full clauses.

Because Russian has no articles.

So Russian does not normally have separate words for a/an or the. Whether something is definite or indefinite is understood from:

• context
• word order
• intonation
• general situation

That means фермер can mean a farmer or the farmer, depending on context. The same is true for конюшне, лошади, and овёс.