Learn Hardcore Finnish: Minun täytyy varata aika lääkärille huomenna. - I must book an appointment with the doctor tomorrow.

In this necessity construction, it is almost always used in 3rd person singular:
- minun täytyy
- sinun täytyy
- meidän täytyy, etc.
  The form täytyy itself does not change with person.
The person is shown only by the genitive pronoun (minun, sinun, meidän...).

Other tenses:

Past: minun täytyi varata – I had to book
Conditional: minun täytyisi varata – I should / ought to book

So it is a verb, but in this pattern you just memorize it as [genitive] + täytyy + infinitive.

What form is varata here, and why isn’t it conjugated?

Varata is the basic infinitive form (the dictionary form of the verb).

After täytyy, Finnish uses the infinitive of the main action:

Minun täytyy varata – I must book
Sinun täytyy mennä – You must go
Meidän täytyy opiskella – We must study

If you conjugated varata (varaan, varaat, varaa...), it would clash with the structure. The finite verb here is täytyy; varata stays as an infinitive.

Why is it aika and not aikaa or ajan?

Aika is the object of varata.

Varata aika = to book an appointment (a time slot)

In affirmative sentences, when you are talking about a whole, complete object, Finnish usually uses the total object, which here looks like the basic form aika.

Compare:

Minun täytyy varata aika. – I have to book an appointment (a full, specific one).
Minun ei tarvitse varata aikaa. – I don’t have to book an appointment (partitive aikaa in negation).

So aika here is the total object form; aikaa (partitive) would appear, for example, in negative sentences or when emphasizing something incomplete or unbounded.

What does aika lääkärille mean as a phrase?

Aika lääkärille is a noun phrase:

aika – time, appointment
lääkärille – to/for the doctor (allative case, -lle)

Together they mean something like “an appointment with the doctor” or “a time (slot) for the doctor”.

The pattern aika + [professional]lle is common:

aika hammaslääkärille – a dentist appointment
aika kampaajalle – a hairdresser’s appointment

Why is it lääkärille with -lle and not lääkärin or lääkärissä?

Lääkärille is in the allative case (ending -lle), which often means:

to someone/someone’s place
for someone
or, in this “appointment” context, with someone (as the professional you see)

In expressions with aika, you normally use allative:

aika lääkärille – appointment with the doctor
aika kampaajalle – appointment with the hairdresser
aika psykologille – appointment with the psychologist

Alternatives would have different meanings:

lääkärin – genitive (“of the doctor”); e.g. lääkärin huone – the doctor’s room
lääkärissä – inessive (“at the doctor’s / at the doctor” physically, inside)

For booking an appointment, aika lääkärille with -lle is the standard idiomatic choice.

Can I leave out minun and just say Täytyy varata aika lääkärille huomenna?

Yes, you can.

Omitting the pronoun is common in Finnish if the person is clear from context. Then it often sounds like a generic or context-dependent “(I/you/we) have to”:

Täytyy varata aika lääkärille huomenna.
→ Depending on context, this is often understood as “I/we really have to book a doctor’s appointment tomorrow.”

If you want to be very explicit that it is you who must do it, keep Minun; otherwise dropping it is natural in casual speech.

Can I change the word order, for example Huomenna minun täytyy varata aika lääkärille?

Yes. Finnish word order is flexible, and adverbs like huomenna (tomorrow) move around quite freely.

Some natural variants:

Minun täytyy varata aika lääkärille huomenna. (neutral)
Huomenna minun täytyy varata aika lääkärille. (slight emphasis on tomorrow)
Minun täytyy huomenna varata aika lääkärille. (emphasis that tomorrow is when the booking should happen)

All of these are grammatical; the differences are mostly about emphasis and flow, not about basic meaning.

How would this sentence look with different persons (you, he/she, we, etc.)?

The pattern stays the same: [genitive pronoun] + täytyy + varata...

Minun täytyy varata aika lääkärille huomenna. – I
Sinun täytyy varata aika lääkärille huomenna. – You (singular)
Hänen täytyy varata aika lääkärille huomenna. – He / she
Meidän täytyy varata aika lääkärille huomenna. – We
Teidän täytyy varata aika lääkärille huomenna. – You (plural / formal)
Heidän täytyy varata aika lääkärille huomenna. – They

Note again that täytyy itself doesn’t change; only the pronoun changes.

What is the difference between minun täytyy varata and minun pitää varata?

Both are very common and usually translate as “I have to / must book”.

Minun täytyy varata aika lääkärille huomenna.
Minun pitää varata aika lääkärille huomenna.

In everyday speech, they are often interchangeable. Very roughly:

täytyy can feel like a strong necessity or inevitability.
pitää can sometimes feel more like an obligation, requirement, or rule.

But the difference is subtle, and in this sentence most Finns would not think there is any real difference in meaning.

Why is huomenna at the end, and does its position change the meaning?

Huomenna is a time adverb (“tomorrow”), and placing it at the end is very normal in Finnish:

Minun täytyy varata aika lääkärille huomenna.

You can move it without changing the core meaning:

Huomenna minun täytyy varata aika lääkärille. (emphasis on tomorrow)
Minun täytyy huomenna varata aika lääkärille. (slight emphasis that the action of booking must occur tomorrow)

So position affects emphasis and rhythm, not the basic meaning “tomorrow”.

How do I say “I had to book an appointment with the doctor” or “I will have to book…”?

Use different tenses of täytyä:

Past (I had to):

Minun täytyi varata aika lääkärille eilen.
– I had to book a doctor’s appointment yesterday.

Future-like meaning (I will have to):

Finnish usually uses the present tense + a time word for future:

Minun täytyy varata aika lääkärille huomenna.
– I will have to book a doctor’s appointment tomorrow.

Context and huomenna (“tomorrow”) carry the future sense.

How do I say “I don’t have to book a doctor’s appointment tomorrow”?

Finnish normally switches from täytyy to tarvita in the negative:

Minun ei tarvitse varata aikaa lääkärille huomenna.
= I don’t have to book a doctor’s appointment tomorrow.

Notes:

Negative of täytyä (ei täyty) is practically not used in this sense.
The object becomes aikaa (partitive) in negation:
- varata aika (affirmative, total object)
- ei tarvitse varata aikaa (negative, partitive object)