Napravi čvrst čvor da se vrećica ne otvori.

Napravi is the imperative singular of the verb napraviti.

It is used when telling one person to do something. In this sentence, it gives a direct instruction.

A useful contrast:

• napravi = tell one person
• napravite = tell several people, or one person politely

Also, napraviti is a perfective verb, so it fits the idea of doing the action as a completed whole: make one proper knot.

Because čvrst has to agree with čvor in:

• gender
• number
• case

Here, čvor is:

• masculine
• singular
• direct object

With a masculine inanimate noun like čvor, the accusative singular looks the same as the nominative singular. So you get:

• čvrst čvor

not a changed ending such as čvrstu čvor or čvrsta čvor.

This is a very common Croatian pattern.

For masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: čvor
• accusative: čvor

That is why after napravi you still see čvor, not a different form.

Compare that with masculine animate nouns, where accusative usually matches genitive instead.

Here da introduces a purpose clause.

So da se vrećica ne otvori means something like:

• so that the bag doesn’t open
• in order for the bag not to open

In this sentence, da is not just a simple that. It shows the purpose of making the knot.

Because Croatian often uses otvoriti se for something that opens on its own or gets opened.

So:

• otvoriti = to open something
• otvoriti se = to open

In this sentence, the bag is not opening something else. The bag itself is the thing that might open, so se is needed.

Without se, the meaning would shift toward opening something else, which would not fit here.

Because vrećica is the subject of the second clause.

The structure is basically:

• the bag opens
• or here, with negation: the bag does not open

So vrećica is not the object of otvori. It is the thing doing the grammatical action of opening, which is why it stays in the nominative:

• vrećica

not:

• vrećicu

No. Here otvori is not an imperative.

In da se vrećica ne otvori, it is the 3rd person singular present form of the perfective verb otvoriti se.

This is a pattern learners see a lot in Croatian:

• after da, Croatian often uses a present-tense form
• with a perfective verb, that can refer to a future or possible event

So da se vrećica ne otvori means so that the bag will not open / doesn’t open, not a command like open!

This is mostly because of Croatian clitic placement.

The short word se is a clitic, and clitics tend to come very early in the clause, often in the second position. After da, it is natural to place se near the front:

• da se vrećica ne otvori

Also, ne normally goes directly before the verb:

• ne otvori

So the order is:

• da
• se
• vrećica
• ne otvori

Other word orders can be possible in Croatian, but this one is very natural.

Yes, vrećica is historically a diminutive form.

It comes from vreća, and -ica often makes a noun smaller or more everyday-sounding.

In practice, though, vrećica is very often just the normal word for things like:

• a small bag
• a plastic bag
• a pouch-like bag

So even though it has a diminutive form, it does not always sound especially cute or emotional.

Croatian does not have articles like English a and the.

So čvor can mean:

• a knot
• the knot

and vrećica can mean:

• a bag
• the bag

The exact meaning depends on context. Croatian speakers rely on context, word order, and emphasis instead of articles.

Yes. Croatian often says napraviti čvor, literally make a knot, and that is completely natural.

You can also hear:

• zavezati čvor = to tie a knot

The difference is small:

• napraviti čvor focuses on creating the knot
• zavezati čvor focuses more on the act of tying

In this sentence, napravi čvrst čvor sounds very natural.