Muszę pilnować psa mojej siostry, bo ten mały pies czasem ucieka z ogrodu.

Muszę means I must / I have to.

After musieć (to have to / must), Polish normally uses another verb in the infinitive:

• muszę pracować = I have to work
• muszę iść = I have to go
• muszę pilnować = I have to watch / keep an eye on

So Muszę pilnować psa... literally means I must watch the dog...

This is a very common question, because the verb pilnować takes the genitive case, not the accusative.

The basic dictionary form is:

• pies = dog

But after pilnować, it changes to:

• pilnować psa = to watch the dog

This is just how the verb works in Polish. Some verbs require specific cases, and pilnować is one of the verbs that commonly takes the genitive.

Compare:

• Widzę psa = I see the dog
• Pilnuję psa = I am watching the dog / keeping an eye on the dog

In both examples you see psa, but for different grammatical reasons. With pilnować, learners often simply memorize the pattern:

• pilnować kogo? czego? = to watch / guard whom? what?

My sister's dog in Polish is literally the dog of my sister:

• pies mojej siostry

In your sentence, because pies has already changed to psa after pilnować, you get:

• pilnować psa mojej siostry = to watch my sister's dog

Here is the breakdown:

• moja siostra = my sister
• mojej siostry = of my sister

Both words change because siostry is in the genitive singular, and mojej must agree with it.

So:

• moja siostra = nominative
• mojej siostry = genitive

This is the normal way to express possession in Polish when English uses 's.

Polish does not use an apostrophe construction like English John's, my sister's, etc.

Instead, possession is usually shown with the genitive case:

• samochód brata = my brother's car / the car of my brother
• dom rodziców = my parents' house
• pies mojej siostry = my sister's dog

So English uses word order plus 's, while Polish usually uses a noun phrase with the possessor in the genitive.

Ten literally means this or sometimes that, depending on context.

Here, ten mały pies is referring back to the dog already mentioned. In English, we would often just say:

• ...because the little dog sometimes runs away from the garden
• or ...because that little dog sometimes runs away from the garden

In Polish, ten is often used anaphorically, meaning it points back to something already known from the conversation. It can feel a bit like the in some contexts, even though Polish has no articles.

So in this sentence:

• bo ten mały pies... = because this/that little dog...

It helps identify the dog clearly and can add a slightly familiar or expressive tone.

Polish could use a pronoun, but repeating the noun is often very natural, especially when the speaker wants to sound clear, vivid, or slightly emphatic.

So:

• bo ten mały pies czasem ucieka z ogrodu
  feels natural and descriptive.

A version with a pronoun is also possible:

• Muszę pilnować psa mojej siostry, bo on czasem ucieka z ogrodu.

That version is grammatically fine, but it is less vivid. The original sentence highlights the dog a bit more, almost like:

• ...because that little dog sometimes runs away from the garden.

Bo means because and is very common in everyday Polish.

So:

• Muszę pilnować psa mojej siostry, bo... = I have to watch my sister's dog because...

You could also say ponieważ, which also means because, but it is usually a bit more formal or neutral in tone.

Compare:

• bo = common, conversational
• ponieważ = a bit more formal

In this sentence, bo sounds very natural.

Ucieka is the 3rd person singular present tense of the imperfective verb uciekać.

• uciekać = to run away / to escape (imperfective)
• ucieka = he/she/it runs away / is running away

Because the sentence says czasem (sometimes), the idea is habitual or repeated action. That is why Polish uses the imperfective:

• czasem ucieka = sometimes runs away

Other forms would mean something different:

• uciec = infinitive, to run away
• ucieknie = perfective future, will run away

So:

• Ten pies czasem ucieka. = This dog sometimes runs away.
• Ten pies ucieknie. = This dog will run away.

After z meaning from / out of, Polish normally uses the genitive case.

The basic form is:

• ogród = garden

After z, it becomes:

• z ogrodu = from the garden / out of the garden

So:

• ucieka z ogrodu = runs away from the garden

This is the standard pattern:

• z domu = from the house
• z pokoju = from the room
• z ogrodu = from the garden

Od also can mean from, but it is used differently. Od ogrodu would not fit naturally here. With movement out of an enclosed or defined place, z is the normal choice.

It can suggest either, depending on context.

Literally, z ogrodu means from the garden or out of the garden. In this sentence, the natural sense is probably:

• the little dog sometimes runs out of the garden
• or escapes from the garden

English chooses the most natural wording based on context, but the Polish structure is straightforward:

• ucieka = runs away / escapes
• z ogrodu = from / out of the garden

Czasem means sometimes.

Its position is fairly natural here:

• ten mały pies czasem ucieka z ogrodu

This is similar to English that little dog sometimes runs away from the garden.

Polish word order is more flexible than English, so czasem could move in some contexts, but the original order is very normal and neutral. It places the frequency word right before the verb, which makes the meaning easy to follow.

Yes, that is possible, but it changes the feel slightly.

• bo mały pies... = because the little dog...
• bo ten mały pies... = because this/that little dog...

Adding ten makes the reference a bit more specific and slightly more natural in many contexts when you are talking about a dog already mentioned. It can sound more like you are pointing back to that particular dog.

So both are possible, but the original version with ten is very natural and expressive.