Na vezu polako popusti konop, spusti jedro i priveži jahtu.

In nautical Croatian, vez means a berth, mooring place, or the state of being moored. So na vezu means something like at the berth, at the mooring, or when mooring.

It helps set the situation for the commands that follow.

Because na can take different cases, and here it is followed by the locative form vezu.

With na + locative, Croatian usually expresses location or state:

• brod je na vezu = the ship is at the berth / moored

So na vezu is a fixed and very common nautical expression.

Yes. They are all imperative forms, meaning they give instructions or commands.

Here they are all 2nd person singular imperatives:

• popusti = loosen / ease
• spusti = lower
• priveži = tie up / moor

So the sentence is speaking to one person in an informal singular way, or giving a general instruction as if addressing one operator.

Because Croatian usually drops subject pronouns when they are not needed.

The verb form already tells you who is being addressed:

• popusti
• spusti
• priveži

all clearly mean you (singular), do this.

If you wanted a plural or formal version, you would use:

• popustite
• spustite
• privežite

The two verbs are similar in form, but they mean different actions:

• popustiti = to loosen, to slacken, to ease off
• spustiti = to lower, to bring down

So:

• popusti konop = slacken/ease the rope
• spusti jedro = lower the sail

You would not normally use spusti for the rope here, because the point is not to lower the rope but to let it out / loosen it gradually.

All three are direct objects, so they are in the accusative case. But their forms look different because they belong to different noun classes.

• konop is masculine inanimate

  • nominative: konop
  • accusative: konop
    For masculine inanimate nouns, accusative is usually the same as nominative.

• jedro is neuter

  • nominative: jedro
  • accusative: jedro
    Neuter nominative and accusative are usually the same.

• jahta is feminine

  • nominative: jahta
  • accusative: jahtu
    Feminine nouns in -a usually change to -u in the accusative singular.

Because Croatian imperatives are often built from the verb’s present stem, not simply by cutting the infinitive and adding an ending.

The infinitive is privezati.
But the present forms are like:

• privežem
• privežeš

So the imperative becomes:

• priveži

This kind of stem change is common. Compare:

• rezati → reži
• vezati → veži
• pokazati → pokaži

So priveži is the normal and correct imperative form.

Because this sentence gives a sequence of completed actions to carry out:

• loosen the rope
• lower the sail
• moor the yacht

In Croatian, perfective verbs are very common in instructions when the speaker wants the action to be done once and to completion.

Here:

• popustiti
• spustiti
• privezati

are all perfective.

If you used imperfective forms, the meaning would shift more toward an ongoing or repeated process, for example:

• popuštaj
• spuštaj
• privezuj

Those would sound more like keep loosening, be lowering, tie repeatedly / keep tying, which is not the main point here.

Polako means slowly, gently, or carefully, depending on context.

In this sentence it most naturally modifies popusti:

• polako popusti konop = slowly/gently ease the rope

It suggests that the rope should not be released suddenly or roughly.

Yes. Croatian word order is fairly flexible.

The sentence begins with Na vezu to set the situation first, and then gives the commands. That is natural and idiomatic.

Polako comes before popusti for emphasis, but other orders are also possible, for example:

• Na vezu popusti konop polako, spusti jedro i priveži jahtu.

That said, the original order sounds clear and natural for an instruction.

Because Croatian often lists commands the same way English lists actions:

• popusti konop, spusti jedro i priveži jahtu

This is just a normal series:

1. loosen the rope
2. lower the sail
3. tie up the yacht

The final i means and, just like in English.