Jutro idę do dentysty, bo od rana bolą mnie zęby, a moja siostra już była u dentystki.

In Polish, the present tense is often used for a planned or arranged future action, especially with verbs of motion like iść.

So Jutro idę do dentysty naturally means Tomorrow I’m going to the dentist.

The time word jutro already makes the future meaning clear, so Polish does not need a separate future form here.
You could also say Jutro pójdę do dentysty, but that sounds a bit more like I will go as a one-time future action, while idę often feels more like a plan or appointment.

Because the preposition do requires the genitive case.

The basic dictionary form is dentysta, but after do, it changes to dentysty.

So:

• dentysta = dentist
• do dentysty = to the dentist

This is very common in Polish: prepositions often force a noun into a particular case.

These are two different nouns:

• dentysta = a male dentist, or a generic dentist
• dentystka = a female dentist

In the sentence:

• idę do dentysty can mean I’m going to the dentist, often using the masculine form generically
• moja siostra już była u dentystki specifically says that the sister went to a female dentist

So this is partly about gender, and partly about case:

• dentysta → dentysty
• dentystka → dentystki

They use different prepositions with different meanings:

• do
  • genitive usually means to, showing movement toward a place or destination
• u
  • genitive usually means at someone’s place, with someone, or visiting a professional

So:

• idę do dentysty = I’m going to the dentist
• była u dentystki = she was at the female dentist’s / she visited the female dentist

With doctors, dentists, hairdressers, and similar professionals, u is very common when talking about being there or having had a visit.

Because the subject is zęby, which is plural.

The verb must agree with the subject:

• ząb boli = a tooth hurts
• zęby bolą = teeth hurt

So bolą mnie zęby literally works like my teeth hurt me.

This is different from English, where we often say my teeth hurt without thinking about this structure. In Polish, the plural subject zęby controls the plural verb bolą.

Because the verb boleć usually takes the person feeling the pain in the accusative case.

So:

• mnie = me
• cię = you
• go/ją = him/her

That is why Polish says:

• Boli mnie głowa = My head hurts
• Bolą mnie zęby = My teeth hurt

Using mi here would be wrong, because mi is dative, and boleć does not normally use dative for this pattern.

Because in Polish, with body parts and things closely connected to a person, the possessive is often omitted if it is already clear who they belong to.

Here, mnie already tells us whose teeth are hurting. So bolą mnie zęby naturally means my teeth hurt.

You could say bolą mnie moje zęby, but that usually sounds unnecessary, heavy, or specially emphasized.

The singular is ząb and the nominative plural is zęby.

This is a normal stem change in Polish. Some nouns change their vowel when the form changes.

So:

• ząb = tooth
• zęby = teeth
• zębów = of teeth

It is just something you need to learn as part of the noun’s pattern.

Od rana means since the morning, from early morning, or all morning depending on context.

In this sentence, it means the tooth pain started in the morning and has been continuing.

Compare:

• rano = in the morning
• od rana = since morning / from the morning onward

So od rana bolą mnie zęby suggests an ongoing problem.

Już means already.

It shows that the sister’s visit happened before now, and it creates a contrast with the speaker, who is only going tomorrow.

So:

• moja siostra była u dentystki = my sister was at the female dentist’s
• moja siostra już była u dentystki = my sister has already been to the female dentist

Because siostra is feminine, and Polish past tense agrees with the gender of the subject.

So:

• siostra była = sister was
• brat był = brother was

This is a very important feature of Polish past tense: you often need to know whether the subject is masculine, feminine, or plural.

In this sentence, a works like and, while, or whereas, with a slight sense of contrast.

The sentence compares two related facts:

• I’m going tomorrow
• my sister has already gone

So a is a good choice because it links the clauses while also showing a contrast or shift in topic.

If you used i, it would sound more like simple addition, with less contrast.

Bo means because.

It is very common in everyday spoken and written Polish. In this sentence, it introduces the reason for going to the dentist:

• idę do dentysty, bo... = I’m going to the dentist because...

You could also use ponieważ, which also means because, but ponieważ sounds a bit more formal or written.
So here bo is the most natural everyday choice.

No, Polish word order is fairly flexible, because the cases and verb forms already show the grammatical relationships.

This sentence uses a very natural order, but some parts could move for emphasis. For example:

• Od rana bolą mnie zęby
• Zęby bolą mnie od rana

Both are possible, though the first is more natural.

Putting jutro first highlights the time: Tomorrow is the important starting point of the sentence.