De thermostaat regelt de verwarming.

Because the subject is de thermostaat, which is a singular noun, and in the present tense Dutch usually adds -t to the verb for hij/zij/het and for singular noun subjects.

The infinitive is regelen.
Present tense:

• ik regel = I regulate/control
• jij regelt = you regulate/control
• hij/zij/het regelt = he/she/it regulates/controls

So:

• De thermostaat regelt de verwarming.

This is very similar to English he regulates / it controls, where English also adds an extra ending in the third person singular.

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

• subject
  • finite verb
    • object

So here:

• De thermostaat = subject
• regelt = finite verb
• de verwarming = direct object

This happens to match standard English word order in a sentence like this, so it feels familiar.

In Dutch, singular nouns usually take either de or het.

Both of these nouns are de-words:

• de thermostaat
• de verwarming

For a learner, the most important thing is to memorize the article with the noun:

• de thermostaat
• de verwarming

A useful note:

• de is used for most singular nouns of common gender
• het is used for neuter nouns
• all plural nouns take de

Yes. Verwarming comes from verwarmen, which means to heat or to warm up.

The ending -ing often creates a noun from a verb, a bit like English endings such as -ing or nouns like heating.

So:

• verwarmen = to heat
• de verwarming = the heating / the heating system

In this sentence, de verwarming usually means the heating system, not just the abstract idea of heat.

Here regelen means something like:

• to regulate
• to control
• to adjust/manage automatically

So De thermostaat regelt de verwarming means the thermostat controls or regulates the heating.

Dutch regelen is a useful verb, but it can also have other meanings depending on context, such as:

• to arrange
• to take care of
• to organize

For example:

• Ik regel het. = I’ll take care of it / I’ll arrange it.

So the exact English translation depends on context.

Not every time. In this sentence, control/regulate works well. But in other contexts, regelen often means arrange or sort out, which English control would not express.

For example:

• Zij regelt een afspraak. = She arranges an appointment.
• Hij regelt het vervoer. = He arranges the transport.

So it is better to think of regelen as a flexible verb whose meaning depends on what is being regulated or arranged.

Because de verwarming is the direct object of the verb regelt.

Dutch, like English, can often use a verb directly with its object:

• De thermostaat regelt de verwarming.
• The thermostat regulates the heating.

No preposition is needed here.

A rough English-friendly guide:

• thermostaat → ther-mo-STAAT
• regelt → roughly RAY-khelt or RAY-guhlt, depending on accent and how carefully it is said
• verwarming → roughly ver-VAHR-ming

A few pronunciation notes:

• aa in thermostaat is a long a sound.
• g in standard Dutch is the throaty Dutch g/ch sound, not the English g in go.
• w in Dutch is usually softer than English w, somewhere between English v and w.
• The stress is usually:
  • thermostaat
  • regelt
  • verwarming

For most learners, the practical answer is: it is a de-word.

In modern Dutch, learners usually focus on:

• de-words
• het-words

Traditional masculine/feminine distinctions still exist in some grammar and in some varieties of Dutch, but for everyday learning, what matters most is that you say:

• de thermostaat

The same goes for:

• de verwarming

No. Unlike German, Dutch does not capitalize all nouns.

So you write:

• De thermostaat regelt de verwarming.

not:

• De Thermostaat regelt de Verwarming.

Only the first word of the sentence and proper names are capitalized.

Yes. In Dutch yes/no questions, the verb usually comes first:

• Regelt de thermostaat de verwarming?

That means:

• Does the thermostat regulate the heating?

This verb-first pattern is very common in Dutch questions.

Yes. That is a passive version:

• De verwarming wordt geregeld door de thermostaat.

It means The heating is regulated by the thermostat.

But the original active sentence:

• De thermostaat regelt de verwarming.

is simpler and more natural in most everyday situations.