De tiende dag begint.

Because dag is a de-word in Dutch. Nouns in Dutch take either de or het as their definite article, and you usually have to learn this together with the noun.

So:

• de dag = the day
• not het dag

There is no simple rule that works for every noun, but many everyday time words are de-words, and dag is one of them.

Tiende means tenth. It is an ordinal number, meaning it shows position in a sequence:

• een = one → eerste = first
• twee = two → tweede = second
• drie = three → derde = third
• tien = ten → tiende = tenth

In this sentence, tiende describes dag, so it works like an adjective: the tenth day.

In Dutch, adjectives and ordinal numbers usually take -e when they come before a noun with de.

So:

• de tiende dag
• de eerste keer
• de vierde maand

This is normal adjective/ordinal agreement in Dutch. Since dag is a de-word, tiende appears with -e.

This is the normal Dutch main-clause word order when the subject comes first:

• De tiende dag = subject
• begint = verb

So it follows the common pattern:

Subject + finite verb

Dutch main clauses also follow the V2 rule, which means the finite verb is in the second position. Here, the subject phrase is first, so the verb comes right after it.

Begint comes from the infinitive beginnen, which means to begin or to start.

Present tense forms are:

• ik begin = I begin
• jij begint = you begin
• hij/zij/het begint = he/she/it begins

In De tiende dag begint, the subject is singular, so Dutch uses the third-person singular form begint.

Because beginnen is the infinitive, meaning to begin. In a normal sentence, Dutch usually uses a conjugated verb, not the infinitive.

Here the subject is de tiende dag, which is singular, so the verb must be singular too:

• beginnen = to begin
• begint = begins

So De tiende dag begint literally matches The tenth day begins.

Yes. In many contexts, beginnen and begint can be translated as either begin or start in English.

So this sentence could naturally be understood as:

• The tenth day begins
• The tenth day starts

Both are fine English translations, depending on style.

Because the sentence is talking about one specific day: the tenth day.

So Dutch uses the singular noun:

• dag = day

If it were plural, it would be dagen:

• de tiende dagen would not make sense here, because tenth points to one item in an ordered sequence.

A rough English-friendly pronunciation is:

duh TIN-duh dakh buh-GINT

A few useful notes:

• de is usually weak and short, like duh
• tiende has the stress on the first syllable: TIN-
• dag has a Dutch a sound that is broader than the English a in day
• g in dag and begint is the Dutch guttural g/ch sound in many accents
• begint has stress on the second syllable: be-GINT

Yes, that is possible, but begint is more neutral and more common in a sentence like this.

• De tiende dag begint = very natural
• De tiende dag start = possible, but a bit less typical for a day beginning

Dutch often prefers beginnen for events, periods, lessons, days, and similar things.

Yes—this sentence is a good example of three useful Dutch patterns:

• Learn nouns with their article: de dag
• Ordinal numbers often end in -de or -ste: tiende
• Verbs change form for the subject: beginnen → begint

So this short sentence helps reinforce:

• article gender
• ordinal numbers
• present-tense verb conjugation