De achttiende week begint.

Because week is a de-word in Dutch. Its singular definite article is de:

• de week = the week
• de weken = the weeks

So in this sentence, de is simply the correct article for week.

Achttiende is the ordinal number eighteenth.

It comes from achttien = eighteen, but when Dutch talks about order or position, it uses an ordinal:

• achttien = eighteen
• achttiende = eighteenth

So de achttiende week means the eighteenth week.

Because achttien is a cardinal number (eighteen), while this sentence needs an ordinal number (eighteenth).

Compare:

• achttien weken = eighteen weeks
• de achttiende week = the eighteenth week

If you are saying which week in a sequence, Dutch uses the ordinal form.

Dutch normally writes number words as one word, including ordinals.

So you get:

• achttien
• achttiende
• tweeëntwintig
• honderdvijf

This is very normal in Dutch spelling. You can also use digits instead, for example de 18e week.

The infinitive is beginnen = to begin / to start.

In the present tense, the singular subject takes the singular verb form:

• ik begin
• jij begint
• hij/zij/het begint

Here, the subject is de achttiende week, which is singular, so the verb is begint.

This is a normal Dutch main clause:

• De achttiende week = subject
• begint = finite verb

Dutch main clauses normally put the finite verb in the second position. Here the subject comes first, so the verb comes right after it.

For example:

• De achttiende week begint.
• Morgen begint de achttiende week.

In both cases, the conjugated verb is in position 2.

Grammatically, begint is present tense. But Dutch often uses the present tense for a scheduled or expected future event, just like English often does.

For example:

• De achttiende week begint morgen.

This is present tense in form, but it refers to the future.

Yes. That is perfectly normal.

Both of these are correct:

• De achttiende week begint.
• De 18e week begint.

The version with digits is especially common in practical contexts such as schedules, calendars, school plans, or reports.

A rough English-friendly guide:

• achttiende ≈ akh-TEEN-duh
• begint ≈ buh-GINT

A few pronunciation notes:

• ch in achttiende is the guttural Dutch sound, like the ch in Scottish loch
• ie sounds like English ee
• the final -e sounds like a weak uh
• g in begint is also guttural in standard Dutch

The hardest part for many English speakers is the rough ch/g sound.

Yes. In many contexts, beginnen can translate as either to begin or to start.

So De achttiende week begint could naturally be understood as:

• The eighteenth week begins
• The eighteenth week starts

Which English word sounds best depends on style, not on a major difference in Dutch meaning.

Yes: ordinals in Dutch behave a lot like adjectives and usually come before the noun:

• de eerste dag = the first day
• het tweede hoofdstuk = the second chapter
• de achttiende week = the eighteenth week

So this sentence follows a very common pattern: article + ordinal + noun + verb.