Ako ne možeš zavezati čvor jednom rukom, stani, udahni i pokušaj ponovno.

Yes. Ako is the basic Croatian word for if, and ne možeš means you cannot / you can’t.

• ako = if
• možeš = you can
• ne možeš = you can’t

Croatian commonly uses the present tense after ako when talking about a real or general condition:

• Ako ne možeš, stani. = If you can’t, stop.

So this is a very normal structure.

This is mainly about aspect, which is very important in Croatian verbs.

• vezati = to tie, be tying, tie in a general/ongoing sense
• zavezati = to tie up, to succeed in tying, to complete the tying

In this sentence, the idea is manage to tie a knot successfully, so zavezati fits better because it suggests a completed action.

Compare:

• Ne mogu vezati. = I can’t tie / I’m unable to do the tying.
• Ne mogu zavezati čvor. = I can’t tie the knot / I can’t get the knot tied.

English often does not show this contrast clearly, but Croatian does.

Čvor is the direct object, so it is in the accusative case.
However, for many inanimate masculine nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: čvor = knot
• accusative: čvor = knot

That is why you do not see a change here.

If it were a masculine animate noun, you would usually see a different form.

Jednom rukom is in the instrumental case, which is often used to mean with something.

So:

• rukom = with a hand / by hand
• jednom rukom = with one hand

Breakdown:

• jedna = one
• rukа = hand
• instrumental singular:
  • jednom
  • rukom

So the phrase literally works like with one hand.

Here it means one, not once.

That can confuse learners because jednom can also mean once in other contexts. But here the grammar makes it clear:

• jednom rukom = with one hand

Because jednom agrees with rukom in case, gender, and number, it is clearly the numeral/adjective form, not the adverb once.

Compare:

• Jednom sam bio tamo. = I was there once.
• To radi jednom rukom. = He/she does that with one hand.

The commas separate parts of the sentence and a series of commands.

Sentence structure:

• Ako ne možeš zavezati čvor jednom rukom, = if you can’t tie a knot with one hand,
• stani, udahni i pokušaj ponovno. = stop, take a breath, and try again.

The first comma separates the if-clause from the main clause.

Then stani and udahni are short imperative verbs listed one after another, so commas help separate them naturally:

• stani = stop
• udahni = inhale / take a breath
• pokušaj ponovno = try again

They are imperatives addressed to you singular.

• stani = stop
• udahni = inhale / take a breath
• pokušaj = try

These are commands or instructions directed at one person, equivalent to English stop, breathe in, try again.

If you were speaking to more than one person, the forms would be different:

• stanite
• udahnite
• pokušajte

Because Croatian often leaves subject pronouns out when the verb already shows who the subject is.

For example:

• možeš already means you can
• stani already means you stop
• udahni already means you inhale
• pokušaj already means you try

So adding ti is usually unnecessary unless you want emphasis or contrast.

Compare:

• Ako ne možeš..., stani... = neutral, normal
• Ako ti ne možeš..., stani... = more emphatic, like if you can’t...

Croatian very often uses the present tense in conditional clauses with ako, even when English might think in a slightly future-oriented way.

So:

• Ako ne možeš..., stani.
  literally: If you can’t..., stop.

This sounds natural in Croatian for instructions, general advice, or real situations. You do not need a special future form here.

Ponovno means again. Yes, opet is also possible in many situations.

So these are both natural:

• pokušaj ponovno
• pokušaj opet

A slight difference in feel:

• ponovno can sound a bit more neutral or slightly more formal
• opet is very common and everyday

In this sentence, ponovno works very well in an instructional or encouraging tone.

Yes, Croatian word order is fairly flexible, though some versions sound more natural than others.

The original sentence is very natural:

• Ako ne možeš zavezati čvor jednom rukom, stani, udahni i pokušaj ponovno.

You could also hear variations such as:

• Ako čvor ne možeš zavezati jednom rukom, stani, udahni i pokušaj ponovno.

That change puts a little more attention on čvor.

Still, the original order is probably the clearest and most neutral for a learner.

Yes, udahni literally means inhale or take a breath in.

The base verb is:

• udahnuti = to inhale

In English, we often say breathe or take a breath in this kind of advice. Croatian can use udahni very naturally in the same calming, instructional way.

So although the literal meaning is closer to inhale, the overall sense is simply:

• stop
• take a breath
• try again

Yes, zavezati čvor is a normal and idiomatic way to say tie a knot.

• čvor = knot
• zavezati čvor = tie a knot

You may also meet related expressions with other verbs depending on context, but zavezati čvor is absolutely standard and natural.