Ahelyett, hogy a fémet beletennénk, külön zsákot használunk.

Yes. Ahelyett, hogy is a fixed Hungarian expression meaning instead of followed by a clause.

• ahelyett literally means something like in place of that
• hogy introduces the following clause

So Hungarian uses a full clause here, not an -ing form like English often does.

Because hogy introduces a subordinate clause, and Hungarian normally separates that clause with a comma.

So:

• Ahelyett, hogy a fémet beletennénk, ...

is standard punctuation.

After ahelyett, hogy, Hungarian usually uses the conditional form.

So although English says instead of putting..., Hungarian prefers a structure that is literally closer to instead of us putting... / instead of we would put...

That does not make the sentence sound especially hypothetical in Hungarian. It is just the normal pattern after ahelyett, hogy.

beletennénk can be broken down like this:

• bele = into it / into that
• tenni = to put
• -nénk = we in the conditional

So the whole form means roughly we would put it into it / in there.

This is a very typical Hungarian way of building meaning into one word.

be means in, into.

bele means into it / into that thing. It usually points more clearly to some container or place already understood from the context.

So:

• betenni = put in
• beletenni = put into it / put in there

In real usage, the difference can be subtle, but bele- often sounds more specific.

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

• fém = metal
• a fém = the metal
• a fémet = the metal as the object of the verb

So here a fémet is the thing being put somewhere.

You also see an extra vowel in fémet. Hungarian often inserts a vowel before -t when needed for pronunciation.

This is a great question, because Hungarian really does have definite and indefinite verb conjugation.

The reason is that in the 1st person plural conditional, the definite and indefinite forms are often identical.

So even though a fémet is definite, tennénk still looks the same.

In other words:

• the sentence does involve a definite object
• but that definiteness is not visibly different in this particular verb form

Because külön zsákot is indefinite: it means a separate bag, not the separate bag.

Hungarian uses:

• indefinite conjugation with indefinite objects
• definite conjugation with definite objects

So:

• külön zsákot használunk = we use a separate bag
• a külön zsákot használjuk = we use the separate bag

That is why használunk is correct here.

külön means separate, distinct, or separately used.

It goes before the noun it modifies:

• külön zsák = a separate bag
• külön zsákot = a separate bag (as object)

So it works much like an adjective here.

Because Hungarian verbs already show the subject.

• használunk = we use
• beletennénk = we would put

So Hungarian usually leaves out the pronoun mi (we) unless it is needed for emphasis or contrast.

Not completely. Hungarian word order is flexible, but different orders change the emphasis.

This sentence is natural because it presents:

1. the instead of clause first
2. the main action after it

So:

• Ahelyett, hogy a fémet beletennénk, külön zsákot használunk.

is a very normal way to say it.

Other orders may also be possible, but they may sound more emphatic or stylistically different.