Čime ćemo se grijati ako opet zahladi?

Čime means with what? or by what means?

It is the instrumental singular form of the question word što. Croatian uses the instrumental case to ask about the means, tool, or substance used for something.

So:

• što = what
• čime = with what / by means of what

In this sentence, the speaker is asking what we will use to keep warm.

Because Croatian forms the future tense with:

• a short form of htjeti = to want
• plus the infinitive

So:

• ćemo grijati = we will heat
• ćemo se grijati = we will keep warm / heat ourselves

The word ćemo is a clitic, which means it usually goes in the second position of the clause.

That is why you get:

• Čime ćemo se grijati...?

and not normally Čime grijati ćemo se...?

Croatian often drops subject pronouns when they are already clear from the verb form.

Here, ćemo already tells you the subject is we, so mi is unnecessary.

• ćemo = we will

You could say Mi ćemo se grijati... if you wanted extra emphasis, but in a normal sentence it is usually omitted.

Se is a reflexive particle. In this sentence, it turns grijati into grijati se.

• grijati = to heat something
• grijati se = to warm oneself, keep warm, be heated

So se grijati does not literally have to mean physically heating your own body with your own hands. Very often it simply means to keep warm or to have heating.

This is a very important distinction:

• grijati = to heat something
• grijati se = to heat oneself, to keep warm, to be heated

Examples:

• Grijem vodu. = I am heating water.
• Grijem se. = I am warming myself / keeping warm.

In your sentence, the meaning is not that we will heat something, but that we will keep warm / heat ourselves.

Zahladi means it gets cold, it turns colder, or the weather cools down.

It comes from the verb zahladiti. In this context, it refers to a change in weather.

So:

• hladno je = it is cold
• zahladi = it becomes cold / it gets colder

Croatian does not need a dummy subject like English it, so just zahladi is enough.

Because after ako = if, Croatian normally does not use the future tense in the same way English does.

English says:

• if it gets cold again
• not if it will get cold again in most normal usage

Croatian works similarly, but even more strongly. After ako, Croatian usually uses the present form.

Here, zahladi is the present form of a perfective verb, and in this kind of clause it refers to a future event:

• ako opet zahladi = if it gets cold again

So this is completely normal Croatian.

The subject is not spoken, because with weather verbs Croatian often uses an impersonal construction.

English needs it:

• if it gets cold again

Croatian does not:

• ako opet zahladi

So the verb appears in 3rd person singular, but there is no actual word corresponding to English it.

Yes, to some extent. Croatian word order is flexible, but not random.

The given sentence is very natural:

• Čime ćemo se grijati ako opet zahladi?

You may also hear:

• Ako opet zahladi, čime ćemo se grijati?

That just moves the if-clause to the front.

You can also sometimes rearrange the future construction:

• Čime ćemo se grijati?
• Grijat ćemo se...

But when clitics like ćemo and se are involved, their position follows special rules, so not every reordering sounds natural.

Opet means again.

In this sentence, it modifies zahladi, so the meaning is:

• if it gets cold again

That implies it has already become cold before, and it may happen once more.

If you moved opet elsewhere, the emphasis could shift slightly, so its current position is natural and clear.

This sentence contains both:

• Č in Čime
• ć in ćemo

For many English speakers, this is tricky.

A simple approximation:

• č is a harder ch sound, somewhat like ch in church
• ć is a softer sound, somewhere between t
  • y and a very soft ch

So:

• Čime starts with the harder sound
• ćemo starts with the softer one

Many learners do not hear the difference well at first, so this is normal. Still, in standard Croatian they are considered different sounds.

It expects an answer naming the means or source of heating.

For example, someone might answer with things like:

• Drvima. = With wood.
• Plinom. = With gas.
• Strujom. = With electricity.

So grammatically, the question invites an answer in the instrumental case, because čime asks with what?