De zesde dag begint.

De is the definite article, meaning the.

In De zesde dag begint, de goes with zesde dag = the sixth day.

A learner might also wonder why it is de dag and not het dag. That is because dag is a de-word in Dutch:

• de dag = the day
• een dag = a day

Zesde means sixth.

• zes = six
• zesde = sixth

This is an ordinal number. Dutch usually forms ordinals by adding -de or -ste:

• vier → vierde = fourth
• vijf → vijfde = fifth
• zes → zesde = sixth

So de zesde dag means the sixth day.

Begint is the correct form because the subject is de zesde dag, which is third person singular.

The verb is beginnen = to begin / to start.

Present tense:

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

Since de zesde dag is a singular thing, Dutch uses begint.

In this sentence, begint usually means begins or starts.

So:

• De zesde dag begint. = The sixth day begins. / The sixth day is starting.

Dutch simple present often covers ideas that English can express with either:

• the simple present (begins)
• or the present progressive (is beginning)

The exact English translation depends on context.

This is the normal Dutch word order for a basic main clause:

subject + verb

• De zesde dag = subject
• begint = verb

So:

• De zesde dag begint.

That is very similar to English:

• The sixth day begins.

Dutch main clauses usually put the finite verb in the second position, and here the subject takes the first position, so the verb comes right after it.

De dag begint is correct, but it means The day begins.

De zesde begint is usually not natural here, because zesde is an adjective/ordinal and normally needs the noun it describes:

• de zesde dag = the sixth day

If the noun has already been made clear in context, Dutch can sometimes leave it out, but for a learner, the safe and normal form is:

• De zesde dag begint.

In de zesde dag, zesde is an adjective-like word placed before a noun with a definite article (de). In that position, it normally takes an -e ending.

That is why you get:

• de zesde dag

This is part of a broader Dutch pattern where adjectives before nouns often take -e:

• de mooie dag = the beautiful day
• de zesde dag = the sixth day

So the -e here is completely normal.

A rough pronunciation guide for English speakers is:

• De ≈ duh
• zesde ≈ ZESS-duh
  (the middle consonants can be a little tricky in fast speech)
• dag ≈ dakh
  (with a Dutch g/ch sound at the end, not a hard English g)
• begint ≈ buh-GINT

Very roughly: duh ZESS-duh dakh buh-GINT

A few notes:

• z sounds like English z
• g in Dutch is usually a throaty sound, not the English g in go
• the stress in begint is on the second syllable: be-GINT

Yes. In many contexts, beginnen can mean both to begin and to start.

So:

• De zesde dag begint. can be understood as:
• The sixth day begins.
• The sixth day starts.

Both are natural English translations, depending on style and context.

Yes, slightly.

• De zesde dag begint. = The sixth day begins.
• Op de zesde dag begint ... = On the sixth day, ... begins

So op de zesde dag means on the sixth day and introduces a time phrase.

For example:

• Op de zesde dag begint het feest. = On the sixth day, the celebration begins.

Your original sentence is not talking about something happening on the sixth day. It says that the sixth day itself is beginning.