Hyvä energialuokka, uusi tiiviste ja hukkalämmön käyttö säästävät energiaa.

Because the subject is a list of three things:

• hyvä energialuokka
• uusi tiiviste
• hukkalämmön käyttö

Together, they form a plural subject, so the verb must also be plural.

• säästää = saves / to save
• säästävät = save / they save

So Finnish works like English here:

• A good energy class, a new seal, and the use of waste heat save energy.

Energiaa is in the partitive case. In this sentence, that is the normal choice because energy is being talked about as an uncountable substance or as a general amount, not as a complete, limited object.

With many verbs, Finnish uses the partitive object when the action affects something in an ongoing, non-total, or indefinite way. Saving energy is a very typical example.

So:

• energia = nominative base form
• energian = genitive / accusative-like form in some contexts
• energiaa = partitive

Here, säästävät energiaa means save energy, not save all the energy or save a specific complete amount of energy.

The -n marks the genitive case.

The phrase hukkalämmön käyttö literally means the use of waste heat. In Finnish, when one noun specifies another noun in this kind of of relationship, the first noun is often put in the genitive.

So:

• hukkalämpö = waste heat
• hukkalämmön = of waste heat
• käyttö = use

Together:

• hukkalämmön käyttö = use of waste heat

This is a very common structure in Finnish.

Because of consonant gradation and stem change.

The word is:

• hukkalämpö in the basic form

But in the genitive, the stem changes:

• lämpö → lämmön

So:

• hukkalämpö → hukkalämmön

This kind of change is common in Finnish. Another familiar example is:

• katu → kadun
• matto → maton

So you are not just adding -n mechanically; sometimes the word stem changes too.

Käyttö is a noun, not a verb.

It comes from the verb:

• käyttää = to use

And the noun:

• käyttö = use, usage

So in hukkalämmön käyttö, the head of the phrase is the noun käyttö. The whole phrase behaves like a noun phrase, and in this sentence it is one item in the subject list.

That is why the sentence is not saying uses waste heat, but rather the use of waste heat.

Because the whole subject is in the nominative.

Each item in the list is a nominative noun phrase:

• hyvä energialuokka
• uusi tiiviste
• hukkalämmön käyttö

In the first two, the adjective agrees with the noun:

• hyvä energialuokka
• uusi tiiviste

Both adjective and noun are in nominative singular.

In the third phrase, there is no adjective; instead, there is a genitive modifier:

• hukkalämmön käyttö

The main noun käyttö is nominative singular, because it is one item in the subject list.

Because Finnish has no articles.

English must choose between things like:

• a good energy rating
• the good energy rating

Finnish usually leaves that distinction to context.

So:

• Hyvä energialuokka can mean a good energy rating/class or the good energy rating/class, depending on context.

This is very normal in Finnish, and learners usually need to get used to understanding definiteness from the situation rather than from an article.

The sentence uses a very straightforward order:

• subject list + verb + object

So:

• Hyvä energialuokka, uusi tiiviste ja hukkalämmön käyttö = subject
• säästävät = verb
• energiaa = object

This is the most neutral word order for a statement like this.

Finnish word order is more flexible than English, but changing it usually changes the focus or emphasis. For example, putting energiaa earlier could make it more topical or contrastive. The version you have is the plain, natural one.

Because Finnish punctuation in a simple list works much like English:

• items are separated by commas
• the last item is introduced by ja = and

So the structure is:

• hyvä energialuokka, uusi tiiviste ja hukkalämmön käyttö

That is:

1. first item
2. comma
3. second item
4. ja
5. third item

Finnish normally does not use an Oxford comma before ja in a simple list.

Yes. Finnish forms compound nouns very often, much more freely than English.

Here:

• energia = energy
• luokka = class/category

Together:

• energialuokka = energy class / energy rating

English often writes these as separate words or with a slightly different structure, but Finnish commonly combines them into one word. This is something English speakers need to get used to, because many ideas that are separate words in English become one compound in Finnish.

Normally, no. In standard Finnish, a subject made of several items joined by ja takes a plural verb.

So:

• X, Y ja Z säästävät energiaa = correct

Using singular säästää here would sound ungrammatical in normal standard Finnish.

Finnish does sometimes use singular verbs with certain collective or abstract ideas, but not with a clear coordinate list like this one.