Min søster går til tandlægen i næste uge, fordi en tand er meget øm.

Because Danish very often uses the present tense for the future when the time is already clear from the sentence.

So:

• Min søster går til tandlægen i næste uge = My sister is going to the dentist next week

The phrase i næste uge already shows that this is future time, so Danish does not need a special future form here.

You could also say:

• Min søster skal til tandlægen i næste uge

That sounds a bit more like a planned or necessary appointment. But the original sentence is completely natural.

Not necessarily. In this sentence, går just means goes / is going.

Danish gå can sometimes mean actual walking, but it is also used more generally in expressions about going somewhere, especially in everyday speech.

So here:

• går til tandlægen means goes to the dentist
• it does not specifically tell you how she gets there

If you wanted to be specific, you could say:

• kører til tandlægen = drives/goes by vehicle to the dentist
• cykler til tandlægen = bikes to the dentist

Because Danish needs the preposition til here.

Til often means to, and with places, appointments, and certain activities it is very common after gå:

• gå til lægen = go to the doctor
• gå til tandlægen = go to the dentist
• gå til fodbold = go to football practice / play football as an activity

So gå til tandlægen is a fixed, natural expression.

In Danish, when talking about going to the doctor, dentist, hairdresser, and similar services, the definite form is very common.

So:

• gå til tandlægen = go to the dentist
• gå til lægen = go to the doctor

This does not necessarily mean a specific dentist already known in the conversation. It is just the normal Danish way to express this kind of visit.

If you said en tandlæge, it would sound more like a dentist in a more general sense, for example:

• Hun leder efter en tandlæge = She is looking for a dentist

Sometimes, but the meaning changes slightly.

• til tandlægen focuses on going to the appointment
• hos tandlægen focuses more on being at the dentist’s place / with the dentist

Examples:

• Hun går til tandlægen i næste uge = She is going to the dentist next week
• Hun er hos tandlægen nu = She is at the dentist now

So in your sentence, til tandlægen is the best choice.

Because in Danish, a possessive like min usually replaces the article.

So:

• min søster = my sister
• søsteren = the sister

You normally do not combine them as min søsteren.

This is the standard pattern:

• min bror = my brother
• mit hus = my house
• mine venner = my friends

There is also a structure like søsteren min, but that is a different word order and often sounds more emphatic or colloquial.

Because en tand introduces one tooth, without identifying which specific tooth has already been mentioned.

So:

• en tand = a tooth
• tanden = the tooth

In the sentence, the speaker is simply saying that one tooth is very sore, not referring back to a previously mentioned tooth.

If the tooth had already been identified, then tanden would make sense.

Tand is a common gender noun, so it takes en in the indefinite singular.

Its main forms are:

• en tand = a tooth
• tanden = the tooth
• tænder = teeth
• tænderne = the teeth

The plural is irregular, so it is worth memorizing:

• singular: tand
• plural: tænder

Øm means something like sore, tender, or sensitive.

So en tand er meget øm means the tooth is painful or tender, especially if touched or used.

It is slightly different from words like:

• ond = bad / evil, but in some expressions related to pain
• gør ondt = hurts

For a tooth, øm is very natural if the tooth feels sore or sensitive.

Because fordi introduces a subordinate clause, and in standard written Danish that clause is separated with a comma.

So the sentence has:

• main clause: Min søster går til tandlægen i næste uge
• subordinate clause: fordi en tand er meget øm

That is why the comma appears before fordi.

Yes. Fordi introduces a subordinate clause, and subordinate clauses follow Danish subordinate-clause word order.

In your sentence:

• fordi en tand er meget øm

There is no extra adverb like ikke, so the word order looks fairly similar to English.

But if you add a sentence adverb, you can see the difference more clearly:

• fordi en tand ikke er øm = because a tooth is not sore

Notice that ikke comes before the finite verb er in the subordinate clause.

That is a useful rule to remember:

• in main clauses, the verb usually comes early because of the V2 rule
• in subordinate clauses, adverbs like ikke usually come before the finite verb

Here i is part of a time expression meaning in / during / next in a natural English translation.

So:

• i næste uge = next week

Danish often uses i with time periods:

• i morgen = tomorrow
• i næste måned = next month
• i år = this year

So although English just says next week, Danish often says i næste uge.

Yes, absolutely.

You can say:

• I næste uge går min søster til tandlægen, fordi en tand er meget øm.

That is also correct and natural. It simply puts more focus on the time.

Notice what happens when the time phrase comes first: Danish keeps the verb in second position, so you get:

• I næste uge
  • går
    • min søster

That is a very important Danish word-order rule.