Pod pretpostavkom da je punomoć valjana, punomoćnik potpisuje vlastoručno.

Pod pretpostavkom da means assuming that, on the assumption that, or provided that in a formal style.

It is a fixed expression:

• pod = under
• pretpostavkom = assumption

So literally it is something like under the assumption that.

This phrase is common in formal, legal, or administrative Croatian. In more everyday Croatian, you might also see:

• ako = if
• ako pretpostavimo da = if we assume that

In your sentence, Pod pretpostavkom da je punomoć valjana... introduces a condition or assumption before the main statement.

Because after pod in this expression, Croatian uses the instrumental case.

The basic noun is:

• pretpostavka = assumption

Its instrumental singular form is:

• pretpostavkom

So:

• pod pretpostavkom = under the assumption

This is a set phrase, so it is best learned as a whole:

• pod pretpostavkom da...

These two words are related, but they mean different things:

• punomoć = power of attorney, authorization
• punomoćnik = attorney-in-fact, authorized representative, the person who acts on the basis of the power of attorney

So in the sentence:

• punomoć is the document/authorization
• punomoćnik is the person who signs

This distinction is very important in legal Croatian.

Je is the 3rd person singular present tense of biti (to be).

So:

• punomoć je valjana = the power of attorney is valid

Inside the clause:

• da je punomoć valjana = that the power of attorney is valid

Croatian often uses da followed by a finite verb in constructions like this.

Because it agrees with punomoć in gender, number, and case.

• punomoć is a feminine singular noun
• so the adjective must also be feminine singular
• valjan = valid
• valjana = valid (feminine singular)

So:

• valjana punomoć = a valid power of attorney
• punomoć je valjana = the power of attorney is valid

In Croatian, grammatical gender is a property of the noun and does not always match what an English speaker might expect.

Punomoć is a feminine noun. One clue is that many feminine nouns end in a consonant in Croatian, especially abstract or formal nouns, though this is something you often just learn with the word.

Because punomoć is feminine, words that agree with it must also be feminine:

• valjana punomoć
• not valjan punomoć

Yes. Potpisuje is the 3rd person singular present tense of potpisivati / potpisati, meaning to sign.

So:

• punomoćnik potpisuje = the representative signs

In legal or formal language, the present tense is often used for rules, procedures, or general statements:

• Punomoćnik potpisuje vlastoručno. = The authorized representative signs by hand / personally signs.

This is not necessarily describing something happening right now. It can describe what is required or what normally happens in a legal procedure.

Because here the sentence says directly who performs the action:

• punomoćnik potpisuje = the representative signs

If you used potpisuje se, that would be a different structure, more like:

• it is signed
• one signs
• or a reflexive/passive-style construction, depending on context

In this sentence, the subject is explicit:

• punomoćnik = the person who signs

So the simple active verb potpisuje is the natural choice.

Vlastoručno means by one’s own hand, personally in handwriting, or handwritten by the person themselves.

It is commonly used in legal and administrative contexts to stress that a signature must be made personally, not mechanically or by someone else.

So:

• potpisuje vlastoručno = signs personally by hand

This word is especially common with:

• vlastoručni potpis = handwritten signature
• vlastoručno potpisati = to sign personally by hand

Here it is an adverb, because it describes how the person signs.

• punomoćnik potpisuje vlastoručno = the representative signs personally/by hand

Compare:

As an adverb:

• potpisuje vlastoručno = signs by hand

As an adjective:

• vlastoručni potpis = a handwritten/personal signature

So in your sentence, it modifies the verb potpisuje.

The first part of the sentence is an introductory subordinate clause or conditional phrase:

• Pod pretpostavkom da je punomoć valjana,
• punomoćnik potpisuje vlastoručno.

Croatian normally separates this introductory clause from the main clause with a comma.

So the structure is:

1. assumption/condition
2. main statement

This is similar to English:

• Assuming that the power of attorney is valid, the representative signs personally.

Punomoćnik is in the nominative singular, because it is the subject of the main clause.

• punomoćnik = the representative
• potpisuje = signs

So:

• Tko potpisuje? = Who signs?
• Punomoćnik.

It is definitely formal, especially legal/administrative Croatian.

Clues include:

• pod pretpostavkom da
• punomoć
• punomoćnik
• vlastoručno

In everyday speech, a Croatian speaker might express the same idea more simply, but in legal documents this wording is very natural and appropriate.

Yes, Croatian word order is fairly flexible, but the given order is natural and formal.

The original:

• Pod pretpostavkom da je punomoć valjana, punomoćnik potpisuje vlastoručno.

Possible variations might exist, but they may sound less neutral or shift emphasis. For example:

• Punomoćnik vlastoručno potpisuje pod pretpostavkom da je punomoć valjana.

That is grammatically possible, but the original is better for formal written style because it presents the condition first and then the main rule.

It is very close in meaning, but not exactly the same in structure.

The whole phrase:

• Pod pretpostavkom da je punomoć valjana

functions like:

• if the power of attorney is valid
• assuming that the power of attorney is valid

So semantically it works like a condition or assumption. But grammatically, Croatian is using the fixed expression pod pretpostavkom da..., not simply ako...

Compare:

• Ako je punomoć valjana, ... = If the power of attorney is valid, ...
• Pod pretpostavkom da je punomoć valjana, ... = Assuming that the power of attorney is valid, ...

The second version sounds more formal and legal.