Tandlægen vil fylde hullet.

Because Danish usually adds the definite article to the end of the noun.

• tandlæge = dentist
• tandlægen = the dentist

This is one of the first big differences from English. Instead of putting the before the noun, Danish often attaches it as a suffix.

Because hul is a neuter noun in Danish.

• et hul = a hole
• hullet = the hole

Danish nouns are mainly divided into two genders:

• en-words: usually take -en in the definite form
• et-words: usually take -et in the definite form

So:

• en tandlæge → tandlægen
• et hul → hullet

Vil is the present tense of ville. In this sentence, it usually means will / is going to.

So Tandlægen vil fylde hullet means something like:

• The dentist will fill the cavity
• The dentist is going to fill the cavity

Depending on context, vil can also sometimes suggest wants to, but here the most natural interpretation is a future action.

Because after a modal verb like vil, Danish normally uses the infinitive without at.

So:

• vil fylde = will fill
• not vil at fylde

This is similar to English:

• will fill
• not will to fill

Other common Danish modal verbs work the same way:

• kan fylde = can fill
• skal fylde = must / will fill
• må fylde = may fill

Fylde is the infinitive form of the verb, meaning to fill.

In this sentence:

• vil is the finite verb
• fylde stays in the infinitive

So the structure is:

• subject
  • modal verb
    • infinitive
      • object

Here:

• Tandlægen = subject
• vil = modal verb
• fylde = infinitive
• hullet = object

Yes, hul literally means hole, but in Danish it is also the normal everyday word for a cavity in a tooth.

So in a dental context:

• hul often corresponds to English cavity
• hullet can mean the cavity

This is a good example of how a literal translation may sound slightly different from the most natural English word.

Yes. This is a very normal basic Danish sentence:

• Tandlægen = subject
• vil = verb in second position
• fylde hullet = rest of the sentence

Danish main clauses usually follow the V2 rule, meaning the finite verb comes in the second position.

In this sentence, the subject comes first, so the order looks straightforward:

• Tandlægen vil fylde hullet.

If you move something else to the front, the verb still stays second:

• I morgen vil tandlægen fylde hullet.
  Tomorrow the dentist will fill the cavity.

A rough English-friendly guide would be:

• tandlægen ≈ TAN-lai-yen or TAN-lay-un

But this is only approximate.

A few helpful points:

• tand contains a soft d in connected speech, so it may not sound like a strong English d
• æ is a front vowel sound, somewhat like the vowel in cat, but not exactly
• the final -gen is often pronounced more softly than an English speaker might expect

Danish pronunciation varies a lot from spelling, so it is best to listen to native audio if possible.

A rough approximation is:

• fylde ≈ FUL-duh or FÜL-duh

But again, that is only a guide.

Useful points:

• y in Danish is not like English y in yes
• it is a rounded front vowel, which many English speakers find difficult at first
• the e at the end is a light unstressed vowel

So fylde may be tricky to pronounce accurately without hearing it.

It could in some contexts, because ville/vil can express wanting as well as future meaning. But in this sentence, especially with a dentist and a cavity, the most likely meaning is:

• will fill
• is going to fill

If you especially wanted to say wants to fill, Danish would often use a clearer wording depending on context, but vil alone can sometimes leave a little ambiguity.

The indefinite version would be:

• En tandlæge vil fylde et hul.

That means:

• A dentist will fill a hole / cavity.

Compare:

• en tandlæge = a dentist
• tandlægen = the dentist
• et hul = a hole / cavity
• hullet = the hole / cavity

The difference is mainly about time or intention.

• Tandlægen vil fylde hullet = The dentist will fill the cavity / is going to fill the cavity
• Tandlægen fylder hullet = The dentist fills / is filling the cavity

The version with vil points more clearly to the future or a planned action.
The version without vil uses the present tense, which in Danish can sometimes also refer to the near future depending on context, but it is less explicit.

Yes, it is complete, grammatical, and natural.

It sounds like a simple statement about what the dentist is going to do. In real conversation, Danish speakers might also use more specific medical or everyday phrasing depending on context, but this sentence is perfectly normal and easy to understand.