Durerea de dinți începe noaptea.

Because durerea is the definite form of durere (pain).

Romanian usually puts the definite article at the end of the noun:

• durere = pain / a pain
• durerea = the pain

So durerea de dinți means the toothache or literally the pain of teeth.

Romanian commonly expresses toothache as durere de dinți or durerea de dinți.

Literally, it is:

• durere = pain
• de = of / related to
• dinți = teeth

So the structure is basically pain of teeth. This is just the normal Romanian way to express the idea.

Because Romanian uses the plural noun in this expression: durere de dinți.

Even though English has toothache with singular tooth, Romanian idiomatically says pain of teeth with plural dinți.

Useful forms:

• dinte = tooth
• dinți = teeth

So this is something to learn as a set phrase.

It is singular.

The main noun is durerea (the pain), and that noun is singular.
de dinți is just a phrase modifying it.

That is why the verb is also singular:

• durerea de dinți începe = the toothache starts

Even though dinți is plural, the whole subject is grammatically singular.

Începe is the 3rd person singular present form of a începe (to begin / to start).

Since the subject durerea de dinți is singular, Romanian uses:

• încep = I start
• începi = you start
• începe = he/she/it starts

So Durerea de dinți începe... means The toothache starts...

In Romanian, expressions of time like at night often use the noun with the definite article:

• noapte = night
• noaptea = the night / at night

Here noaptea functions adverbially and means at night.

This pattern is very common:

• dimineața = in the morning
• ziua = during the day
• seara = in the evening
• noaptea = at night

Romanian often does not need a preposition in these time expressions.

So English at night is simply:

• noaptea

Likewise:

• dimineața = in the morning
• seara = in the evening

This is normal Romanian usage, even though English needs at or in.

Yes, Romanian word order is somewhat flexible.

The neutral order here is:

• Durerea de dinți începe noaptea.

But Romanian could also move noaptea for emphasis:

• Noaptea începe durerea de dinți.

That sounds more marked or literary, though. For learners, the original order is the safest and most natural.

A rough guide:

• dinți ≈ deents
  The ț sounds like ts.

• începe ≈ in-CHEH-peh
  The ce here sounds like che in check.

Also:

• durerea ≈ doo-REH-rea
• noaptea ≈ NOAP-tya

Romanian spelling is fairly consistent, so pronunciation becomes easier with practice.

It can suggest either, depending on context.

Because durerea is definite, it often feels like:

• the toothache starts at night

But Romanian also uses the definite form in fairly general statements, so it can sound natural even when speaking about a typical experience:

• Toothache starts at night.

So the exact nuance depends on the situation, not just the grammar.