De zeventiende afspraak is morgen.

Because zeventiende is an ordinal number: seventeenth.

• zeventien = seventeen
• zeventiende = seventeenth

When you are talking about the position of something in a sequence, Dutch uses the ordinal form, just like English:

• de zeventiende afspraak = the seventeenth appointment

So zeventien afspraak would be incorrect here.

Zeventiende is the ordinal form of zeventien.

Dutch ordinals are usually made with -de or -ste:

• vier → vierde
• acht → achtste
• tien → tiende
• zeventien → zeventiende

So zeventiende simply means seventeenth.

Because it is used before a noun: afspraak.

In Dutch, ordinals used attributively usually appear with -e:

• de eerste dag = the first day
• de tweede keer = the second time
• de zeventiende afspraak = the seventeenth appointment

So here, zeventiende is functioning like an adjective before afspraak, and the -e form is the normal one.

Because afspraak is a de-word.

In Dutch, every noun has a grammatical gender/category for the article:

• de afspraak
• not het afspraak

So when you say the seventeenth appointment, it becomes:

• de zeventiende afspraak

Afspraak most often means appointment, but it can also mean agreement depending on context.

Common uses:

• Ik heb een afspraak met de dokter. = I have an appointment with the doctor.
• Dat is de afspraak. = That is the agreement / that’s what was agreed.

In your sentence, it clearly means appointment.

Because Dutch main clauses usually follow the V2 rule: the finite verb comes in the second position.

Here the sentence is structured like this:

• De zeventiende afspraak = subject
• is = finite verb
• morgen = time expression

So:

• De zeventiende afspraak is morgen.

This is the normal Dutch word order for a simple statement.

Yes, that is grammatically possible, but the emphasis changes.

• De zeventiende afspraak is morgen.
  Focuses first on the seventeenth appointment.

• Morgen is de zeventiende afspraak.
  Focuses first on tomorrow.

Both follow the Dutch V2 rule:

• first element
• finite verb second
• the rest after that

So if morgen comes first, is must still come second.

Here, morgen is an adverb of time, meaning tomorrow.

So:

• morgen = tomorrow

Dutch morgen can also be a noun meaning morning, but then the meaning depends on context, and often you would see other clues:

• de morgen = the morning
• morgenochtend = tomorrow morning

In De zeventiende afspraak is morgen, it clearly means tomorrow.

Yes.

Dutch can use an ordinal by itself when the noun is understood from context:

• de zeventiende = the seventeenth one / the seventeenth

This is also common for dates:

• de zeventiende van mei = the seventeenth of May

But in your sentence, the noun is stated explicitly:

• de zeventiende afspraak

A rough pronunciation guide is:

• ZAY-vuhn-TEEN-duh

More carefully in Dutch IPA, it is often something like:

• /ˌzeː.vənˈtin.də/

A few pronunciation notes:

• z sounds like the z in zoo
• ee is a long vowel
• the stress is mainly on -tien-
• the final -de is pronounced with a soft uh sound

Yes, in some contexts.

• de zeventiende afspraak = the seventeenth appointment
• afspraak nummer zeventien = appointment number seventeen

These are similar, but not always identical in feel:

• de zeventiende afspraak sounds like the item in a sequence
• afspraak nummer zeventien sounds a bit more like labeling or numbering

In many practical situations, both could work.

Yes, it is grammatical and natural.

It is a simple sentence meaning that a particular appointment in a sequence is scheduled for tomorrow. Dutch speakers would understand it immediately.

Depending on context, they might also phrase it differently, for example:

• De zeventiende afspraak is voor morgen.
• Afspraak nummer zeventien is morgen.

But De zeventiende afspraak is morgen is perfectly fine.

Often they do, but not always exactly.

The basic idea is the same:

• eerste = first
• tweede = second
• derde = third
• vierde = fourth

But the forms themselves are different, and you need to learn them in Dutch. Also, when used before nouns, Dutch ordinals normally appear in the -e form:

• de eerste afspraak
• de zeventiende afspraak

So the overall function is similar to English, but the exact word forms are Dutch-specific.