Собака нашла кость во дворе.

Because собака is in the nominative case, which is the normal form used for the subject of a sentence.

In Собака нашла кость во дворе, the verb нашла means found, so собака is the one performing that action.

Russian usually does not use articles like a or the, so собака can mean:

• a dog
• the dog

You understand which one is meant from context.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject собака is:

• singular
• feminine

So the verb must also be feminine singular past:

• нашёл = masculine singular
• нашла = feminine singular
• нашло = neuter singular
• нашли = plural

So Собака нашла... means The dog found...

The dictionary form is найти, which means to find.

нашла is the past feminine singular form of найти.

A very useful pair to know is:

• находить = to be finding / to find repeatedly / imperfective
• найти = to find / to succeed in finding / perfective

In this sentence, нашла is from the perfective verb найти, which suggests a completed result: the dog found the bone.

Here кость is the direct object, so it should be in the accusative case.

The noun кость is:

• feminine
• singular
• inanimate

For many inanimate feminine nouns, the accusative singular looks the same as the nominative singular. So:

• nominative: кость
• accusative: кость

That is why the sentence uses кость.

Yes. In Russian, a noun ending in -ь can be masculine or feminine, so you have to learn its gender.

кость is feminine.

That matters for agreement in other contexts, for example:

• большая кость = a big bone
• эта кость = this bone

So the -ь ending does not automatically tell you the gender by itself.

Both в and во mean in here, but во is used in some places to make pronunciation easier.

дворе begins with the consonant cluster дв-, so во дворе sounds smoother than в дворе.

This is a pronunciation-based choice, not a change of meaning.

Examples:

• в доме = in the house
• во дворе = in the yard

Because after в / во when talking about location, Russian uses the prepositional case.

The noun is двор = yard / courtyard.

Its prepositional singular form is дворе:

• двор = yard
• во дворе = in the yard

So во дворе tells you where the dog found the bone.

This is the difference between location and motion.

• во дворе = in the yard → location, so prepositional case
• во двор = into the yard → motion toward a place, so accusative case

In your sentence, the dog found the bone while being in the yard, so Russian uses во дворе.

Compare:

• Собака была во дворе. = The dog was in the yard.
• Собака побежала во двор. = The dog ran into the yard.

Yes. Russian word order is more flexible than English because case endings show the grammatical roles.

The neutral order here is:

• Собака нашла кость во дворе.

But other orders are possible, for example:

• Во дворе собака нашла кость. = In the yard, the dog found a bone.
• Кость собака нашла во дворе. = It was the bone that the dog found in the yard.

The basic meaning stays similar, but the focus or emphasis changes.

The stress is:

• соба́ка
• нашла́
• кость
• во дворе́

So the whole sentence is pronounced:

Соба́ка нашла́ кость во дворе́.

Stress is very important in Russian, because it is not always predictable.

Russian has no articles.

So a noun like собака can mean:

• a dog
• the dog

And кость can mean:

• a bone
• the bone

Context tells you which meaning is intended. That is completely normal in Russian.

It usually means the dog successfully found the bone.

That is because нашла comes from the perfective verb найти, which focuses on a completed result.

If you wanted to emphasize the process of searching or ongoing action, you would more likely use an imperfective verb, depending on context.

So this sentence naturally means that the dog ended up finding the bone, not just that it was looking for one.