Learn Hardcore Norwegian: Jeg har tannlegetime i morgen, så jeg må ringe tannlegen i dag. - I have a dentist appointment tomorrow, so I have to call the dentist today.

So tannlegetime literally means “dentist appointment”.
In Norwegian, it is correctly written as one word: tannlegetime, not tannlege time.

Why do you say jeg har tannlegetime instead of something like “I will have a dentist appointment”?

Norwegian often uses the present tense to talk about the near future, especially for scheduled events:

Jeg har tannlegetime i morgen.
= I have a dentist appointment tomorrow. (meaning I will have …)

This is very natural in Norwegian and feels like “I’ve got a dentist appointment tomorrow” in everyday English. You can also say jeg skal til tannlegen i morgen, but jeg har tannlegetime is very common and idiomatic.

What is the difference between jeg har tannlegetime i morgen and jeg skal til tannlegen i morgen?

Both can describe a dentist appointment tomorrow, but the focus is slightly different:

Jeg har tannlegetime i morgen.
Emphasizes the appointment itself: “I have a dentist appointment.”
Jeg skal til tannlegen i morgen.
Emphasizes going to the dentist: “I’m going to the dentist.”

In practice, they’re often interchangeable. Har tannlegetime sounds a bit more like “I’ve got a scheduled appointment,” while skal til tannlegen sounds a bit more like focusing on the visit.

Why is har (present tense) used when the appointment is tomorrow?

In Norwegian, the simple present is very commonly used for future events that are fixed or planned:

Jeg har eksamen neste uke.
Vi reiser i juli.
Jeg har tannlegetime i morgen.

English might use “am having”, “will have”, or “have” depending on context, but Norwegian just uses the normal present tense and adds a time word like i morgen / neste uke to show it’s in the future.

What exactly does må mean in jeg må ringe tannlegen?

Må is a modal verb meaning must / have to / need to (obligation or necessity):

Jeg må gå. = I must go / I have to go.
Du må gjøre leksene dine. = You must do your homework.

In jeg må ringe tannlegen, it means I have to call the dentist (because of some necessity, like confirming the appointment).

Why is it må ringe and not må ringer?

Modal verbs in Norwegian (like må, kan, vil, skal, bør) are followed by the infinitive form of the main verb without å:

Jeg må ringe. (not må ringer or må å ringe)
Jeg kan snakke norsk.
Vi skal spise nå.

So the pattern is: [subject] + [modal verb] + [infinitive]
Here: jeg + må + ringe.

Why is it tannlegen (the dentist) and not just tannlege?

Norwegian uses the definite form (tannlegen) when referring to a specific, known dentist, similar to English “the dentist”:

Jeg må ringe tannlegen.
= I must call the dentist (the one I go to).

If you said jeg må ringe en tannlege, it would mean “I must call a (some) dentist”, not a particular one you already have. In this sentence, it’s understood you mean your dentist / the dentist you have an appointment with, so you use the definite form tannlegen.

Is it also correct to say jeg må ringe til tannlegen? Do you need til with ringe?

Both are used:

Jeg må ringe tannlegen.
Jeg må ringe til tannlegen.

Ringe can take a direct object (no preposition) or til + person/place. In everyday speech, many people prefer the short form ringe tannlegen. Adding til can sound slightly more explicit or careful, but both are correct and common.

What are the forms and gender of tannlege (indefinite, definite, plural, etc.)?

Tannlege is usually treated as a masculine noun in Bokmål:

en tannlege = a dentist
tannlegen = the dentist
tannleger = dentists
tannlegene = the dentists

In Bokmål, many masculine nouns can optionally be feminine, so ei tannlege / tannlega is possible in theory, but en tannlege / tannlegen is by far the most standard and common.

Why is the word order …, så jeg må ringe tannlegen i dag and not something like …, så må jeg ringe tannlegen i dag?

In this sentence, så is a coordinating conjunction (like and, but, so in English) linking two main clauses:

Jeg har tannlegetime i morgen,
så jeg må ringe tannlegen i dag.

With coordinating conjunctions (og, men, eller, for, så) Norwegian keeps the normal subject–verb order in the next clause:

…, og jeg går.
…, men jeg blir.
…, så jeg må ringe.

If så is used as an adverb at the beginning of a new sentence (meaning “then / so”), it often comes before the verb:
Jeg var trøtt. Så gikk jeg hjem.
But here it’s a conjunction, so jeg comes first: så jeg må ….

Why are the time expressions i morgen and i dag at the end of each clause? Can they go somewhere else?

Norwegian likes the basic order:

[Subject] – [Verb] – (Other stuff) – [Time]

So:

Jeg har tannlegetime i morgen.
Jeg må ringe tannlegen i dag.

You can also move the time expression to the front for emphasis:

I morgen har jeg tannlegetime.
I dag må jeg ringe tannlegen.

Both are correct. The meaning stays the same; putting time first just highlights it a bit more.

What is the difference between i morgen, på morgenen, and i morgen tidlig?

They talk about different things:

i morgen = tomorrow (the whole day)
på morgenen = in the morning (on a given day)
i morgen tidlig = tomorrow morning (early)

So:

Jeg har tannlegetime i morgen.
= I have a dentist appointment tomorrow (time not specified).
Jeg har tannlegetime i morgen tidlig.
= I have a dentist appointment tomorrow morning (early in the day).
Jeg har tannlegetime på morgenen.
= I have my dentist appointment in the morning (on that day).

How are i dag and i morgen written correctly? Can I write idag or imorgen?

The standard modern spelling in Bokmål is:

i dag (two words)
i morgen (two words)

You may see idag / imorgen in informal writing or older texts, but in correct, standard Norwegian you should write them as two separate words.