A termosztáttal csökkentjük az energiafogyasztást.

Because Hungarian usually does not need a subject pronoun when the verb ending already shows the subject.

Here, csökkentjük means we reduce it:

• csökkent- = reduce
• -jük = we with a definite object

So mi (we) is unnecessary unless you want extra emphasis:

• Mi csökkentjük az energiafogyasztást. = We are the ones reducing energy consumption.

termosztáttal means with the thermostat or by using the thermostat.

The ending is the instrumental/comitative suffix -val/-vel, which often means:

• with
• by means of
• using

So:

• termosztát = thermostat
• termosztáttal = with the thermostat / using the thermostat

In this sentence, it expresses the means by which the action is done.

Because the suffix -val/-vel changes its form after many consonants.

When this suffix is added, the v often assimilates to the final consonant of the noun, and that consonant is written doubled. So:

• termosztát + val becomes termosztáttal

This is a very common spelling pattern in Hungarian:

• kéz + vel → kézzel
• lap + val → lappal
• vonat + val → vonattal

So termosztáttal is exactly what you should expect from Hungarian spelling and pronunciation rules.

Both a and az mean the. The choice depends on the sound at the beginning of the next word:

• a before a consonant
• az before a vowel

So:

• a termosztáttal because termosztát begins with t
• az energiafogyasztást because energiafogyasztást begins with e

This is purely a sound-based rule, like using a vs an in English.

Because Hungarian has two kinds of verb conjugation:

• indefinite conjugation
• definite conjugation

Here the object is definite:

• az energiafogyasztást = the energy consumption

When the direct object is definite, Hungarian normally uses the definite conjugation. That is why we get:

• csökkentjük = we reduce it / we reduce the ...

Compare:

• csökkentünk valamit = we reduce something
• csökkentjük az energiafogyasztást = we reduce the energy consumption

So the verb form is chosen partly because of the object.

That -t is the accusative ending, which marks the direct object.

So:

• energiafogyasztás = energy consumption
• energiafogyasztást = energy consumption as the thing being reduced

In this sentence, az energiafogyasztást is what we reduce, so it takes the accusative.

Because Hungarian often makes compound nouns where English would use a multi-word expression.

Here:

• energia = energy
• fogyasztás = consumption

Together:

• energiafogyasztás = energy consumption

This is very normal in Hungarian. Also, grammatical endings are usually added to the whole compound, at the end:

• energiafogyasztás
• energiafogyasztást

So even though it may look long, it is structurally very regular.

Hungarian word order is more flexible than English word order, and it often reflects topic and focus rather than a fixed subject-verb-object pattern.

Here, A termosztáttal comes first because it sets the frame:

• As for the thermostat
• Using the thermostat
• With the thermostat

Then the rest says what happens:

• csökkentjük az energiafogyasztást = we reduce energy consumption

So this order is natural if the speaker wants to foreground the means used.

Other orders are possible, but they can shift emphasis.

Yes, it could, but the meaning would shift slightly.

• A termosztáttal csökkentjük az energiafogyasztást.
  = with the thermostat / using the thermostat

• Termosztáttal csökkentjük az energiafogyasztást.
  = with a thermostat / by using thermostat control, more generally

With a, the phrase sounds more definite or more connected to a known thermostat. Without the article, it can sound more general or more like talking about the method in general.

Yes. Grammatically, that is a close translation:

• A termosztáttal = with the thermostat
• csökkentjük = we reduce
• az energiafogyasztást = the energy consumption

But in natural English, people often prefer:

• We reduce energy consumption with the thermostat.
• Using the thermostat, we reduce energy consumption.
• The thermostat helps us reduce energy consumption.

So the Hungarian structure is straightforward, even if English may choose a smoother wording.