Odmorite podlakticu tako što koristite naslon za ruke.

Odmorite is the imperative form, meaning rest in the sense of rest/support/relax.

In this sentence, it is:

• 2nd person plural: you all rest
• or polite singular: you rest when addressing one person formally

That is why this kind of form is very common in instructions, manuals, and health/safety advice.

Related forms:

• informal singular: odmori
• plural/formal: odmorite

So a less formal singular version would be:

Odmori podlakticu tako što koristiš naslon za ruke.

Because podlakticu is in the accusative singular.

The base form is podlaktica. It is a feminine noun ending in -a, and feminine nouns of this type usually change like this:

• nominative: podlaktica
• accusative: podlakticu

It is accusative because it is the direct object of odmorite: you rest what? — the forearm.

This is the same pattern as:

• ruka → ruku
• glava → glavu

No. Podlaktica is more specific.

• podlaktica = forearm
  the part between the elbow and the wrist
• ruka = arm or hand, depending on context
• nadlaktica = upper arm

So Croatian often uses a more anatomically precise word where English might sometimes just say arm.

This is mainly about aspect.

• odmoriti is perfective
• odmarati is imperfective

In instructions, Croatian often uses the perfective imperative when the focus is on achieving a result:
put your forearm in a rested/supported position.

So odmorite sounds natural for a single recommended action with a clear result.

If you used odmarajte, it would sound more like:

• keep resting
• rest it repeatedly
• be in the habit of resting it

That is usually not what an instruction like this wants to emphasize.

Tako što means something like:

• by
• by doing
• in such a way that

It introduces the method used to do the action in the main clause.

So the structure is:

Odmorite podlakticu + tako što + koristite naslon za ruke

Literally, this is close to:

• Rest your forearm by using the armrest
• Rest your forearm in this way: use the armrest

This is a very common Croatian pattern for explaining how to do something.

After tako što, Croatian normally uses a finite verb, very often in the present-tense form.

So:

• tako što koristite = by using

English often uses a gerund here, but Croatian usually does not use an infinitive after tako što.

That is why Croatian says:

• tako što koristite naslon za ruke

rather than something built directly like English by to use.

Also, koristite matches the same person/number as the main instruction:

• odmorite → koristite

Formally, koristite can look the same as:

• present 2nd person plural
• imperative 2nd person plural

But in this sentence, after tako što, it is best understood as part of a method clause meaning by using.

So the sentence is not really giving two separate commands:

• Rest your forearm. Use the armrest.

Instead, it is one instruction plus the method:

• Rest your forearm by using the armrest.

Because the two verbs play different roles.

• odmorite uses a perfective verb because the instruction focuses on the result: get the forearm into a rested position.
• koristite uses an imperfective verb because it describes the means/process: you do it by using the armrest.

This combination is very natural in Croatian:

• perfective for the main goal
• imperfective for the ongoing action or method

So the aspect choice is not random; it reflects how the sentence is structured.

Literally:

• naslon = support, rest, leaning surface
• za ruke = for arms/hands

Together, naslon za ruke means armrest.

The preposition za here expresses purpose/function:

• something for the arms

This is a very common pattern in Croatian:

• čaša za vodu = glass for water
• stalak za knjige = stand for books
• naslon za ruke = armrest

It actually is in the accusative, but for this kind of noun the accusative looks the same as the nominative.

The verb koristite takes a direct object:

• koristite što? — naslon za ruke

The noun naslon is:

• masculine
• inanimate

For masculine inanimate nouns, nominative singular and accusative singular are usually identical.

So:

• nominative: naslon
• accusative: naslon

That is why there is no visible ending change.

Because naslon za ruke is the standard name of the object, and that name does not need to match the number of podlakticu.

It is a fixed expression meaning armrest or arm support. The plural ruke is generic: it is the part of a chair designed for the arms.

So Croatian is not saying you are necessarily resting both arms. It is just naming the object you use.

Yes. A common alternative is:

Odmorite podlakticu koristeći naslon za ruke.

That also means Rest your forearm by using the armrest.

The difference is mostly stylistic:

• tako što koristite... sounds very straightforward and explanatory
• koristeći... is a bit more compact and can sound slightly more written or formal

Both are grammatical, but tako što is often especially clear for learners and in practical instructions.