Încălzitorul păstrează apa caldă.

It is the definite article attached to the noun.

• încălzitor = heater / a heater
• încălzitorul = the heater

Unlike English, Romanian usually puts the at the end of the noun instead of using a separate word.

So încălzitorul păstrează... starts with the heater keeps...

It is a neuter noun.

That can be confusing at first, because Romanian neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular you get:

• un încălzitor
• încălzitorul

But in the plural you get feminine-type forms:

• două încălzitoare
• încălzitoarele

So even though încălzitorul looks masculine, the noun itself is actually neuter.

Because apa means the water, while apă means just water or a quantity of water.

• apă = water
• apa = the water

This is the feminine singular definite form. Romanian adds the definite article to the end of the noun:

• apă → apa

So this is not a missing diacritic or a typo. Apa is the correct definite form.

Because the adjective must agree with the noun it describes.

Here, caldă describes apa, and apa is feminine singular, so the adjective must also be feminine singular:

• masculine singular: cald
• feminine singular: caldă

So:

• apa caldă = the warm water

If the noun were masculine singular, you would use cald instead.

Because in Romanian, adjectives often come after the noun.

So the normal order is:

• apa caldă = the warm water

This is one of the big differences from English, where adjectives usually come before the noun.

Romanian can sometimes put an adjective before the noun, but that often sounds more literary, emphatic, or stylistically marked. In a simple sentence like this, apa caldă is the most natural order.

Păstrează is the present tense form of the verb a păstra.

Here it means keeps.

With this subject, it is third person singular:

• încălzitorul păstrează = the heater keeps

A useful detail: păstrează can also be the third person plural form in Romanian, so the exact meaning depends on the subject:

• încălzitorul păstrează = the heater keeps
• încălzitoarele păstrează = the heaters keep

Because Romanian often does not use subject pronouns when they are not needed.

English usually needs a subject pronoun:

• It keeps the water warm.

Romanian can simply say:

• Păstrează apa caldă.

And when the subject noun is already present, as here, adding a pronoun would be unnecessary:

• Încălzitorul păstrează apa caldă.

So Romanian is often more comfortable than English with leaving pronouns out.

The biggest clue is agreement.

• apa is feminine singular
• caldă is also feminine singular

But încălzitorul is singular neuter (which behaves like masculine in the singular), so it would not take caldă in this context.

So Romanian grammar tells you that caldă goes with apa.

Also, the phrase a păstra ceva cald means to keep something warm, so apa caldă fits the structure naturally.

A rough guide is:

în-căl-zi-TOR-ul păs-TREA-ză A-pa KAL-də

A few sounds English speakers usually ask about:

• î = a close central vowel, a sound English does not really have; it is not the same as English ee
• ă = like the a in about
• z is pronounced like English z
• c before ă is a normal k sound

So:

• Încălzitorul begins with that special Romanian î
• păstrează contains both ă sounds
• caldă ends with ă, not a full a

Yes, Romanian word order is somewhat flexible, but this sentence uses the most neutral, natural order:

• Încălzitorul păstrează apa caldă.
• subject + verb + object + adjective

That is the standard way to say it.

You can move things around for emphasis, but then the sentence may sound marked, poetic, or contrastive rather than neutral. For a learner, this version is the best one to copy first.