Ne smiješ pritisnuti tu tipku.

Ne smiješ means you must not, you are not allowed to, or you may not.

It comes from the verb smjeti = to be allowed to / may.

• smiješ = you are allowed
• ne smiješ = you are not allowed

So this sentence is about permission/prohibition, not ability.

Croatian often leaves out subject pronouns when they are already clear from the verb ending.

Here, smiješ already tells you the subject is you (singular).

• smijem = I may
• smiješ = you may
• smije = he/she/it may

So Ne smiješ pritisnuti tu tipku naturally means You must not press that button, even without ti.

You could add ti for emphasis, but it is usually unnecessary:

• Ti ne smiješ pritisnuti tu tipku. = You must not press that button.

Pritisnuti is the infinitive form of the verb, meaning to press.

After smjeti (to be allowed), Croatian commonly uses an infinitive:

• Smijem ući. = I may enter.
• Ne smiješ dirati to. = You must not touch that.
• Ne smiješ pritisnuti tu tipku. = You must not press that button.

So the structure is:

ne smiješ + infinitive

Pritisnuti is a perfective verb, which usually presents the action as a single complete act: to press, to push once, to press and complete the action.

Croatian often uses the perfective infinitive in warnings or prohibitions when talking about a single action:

• Ne smiješ pritisnuti tu tipku. = Don’t press that button / You must not press that button.

A more imperfective verb like pritiskati would suggest repeated or ongoing pressing:

• Ne smiješ pritiskati tu tipku. = You must not keep pressing / press repeatedly on that button.

So pritisnuti fits very well if the idea is don’t press it even once.

Because tipku is in the accusative case, not the nominative.

The verb pritisnuti takes a direct object, and direct objects are often in the accusative.

The noun tipka is feminine singular:

• nominative: ta tipka = that button
• accusative: tu tipku = that button

So:

• Ta tipka je pokvarena. = That button is broken.
  Here tipka is the subject, so nominative.
• Pritisni tu tipku. = Press that button.
  Here tipku is the object, so accusative.

Both the article-like word and the noun change:

• ta → tu
• tipka → tipku

Tu here means that and agrees with tipku.

It is the feminine accusative singular form of taj:

• masculine: taj
• feminine nominative: ta
• feminine accusative: tu
• neuter: to

So:

• ta tipka = that button
• tu tipku = that button (as an object)

This is not the adverb tu meaning here. In this sentence, tu is a demonstrative adjective/pronoun form meaning that.

They are very close, but not exactly identical in tone.

• Ne smiješ pritisnuti tu tipku. = You are not allowed to press that button / You must not press that button.
• Nemoj pritisnuti tu tipku. = Don’t press that button.

The first sounds more like a rule, prohibition, or lack of permission.
The second is a more direct negative command.

In many real situations, both could work.

Because the verb smjeti is about permission.

Compare:

• Ne smiješ pritisnuti tu tipku. = You must not / you are not allowed to press that button.
• Ne možeš pritisnuti tu tipku. = You cannot press that button.

So:

• smjeti = may, be allowed
• moći = can, be able

This distinction is important in Croatian, just as it can be in English.

Yes. Croatian word order is fairly flexible, although some orders sound more natural than others depending on emphasis.

The neutral order here is:

• Ne smiješ pritisnuti tu tipku.

You could also say:

• Tu tipku ne smiješ pritisnuti.
  This puts more emphasis on that button.
• Pritisnuti tu tipku ne smiješ.
  Grammatically possible, but less neutral and more marked.

So the original sentence is the most natural everyday wording unless you want to stress something specific.

Smiješ is pronounced roughly like SMYEHSH, though that is only an approximation.

A few points:

• sm is pronounced together at the start.
• ije is a real vowel sequence in Croatian, not like English spelling.
• š sounds like sh in shoe.

So:

• smiješ ≈ smi-yesh, said smoothly

Depending on accent and region, the exact quality can vary a little, but š = sh is the key thing to remember.

Yes, it can be confusing at first.

• smiješ from smjeti = you may / you are allowed
• smiješ se from smijati se = you are laughing

The reflexive se helps distinguish smijati se:

• Smiješ se. = You are laughing.
• Ne smiješ. = You must not / you are not allowed.

In your sentence, because there is no se and the meaning is about permission, the verb is clearly smjeti.

It normally refers to a specific button: that button.

The demonstrative tu points to something identifiable in the situation:

• a button being shown
• a button already mentioned
• a button the listener can see

If you wanted something more general, Croatian would use a different structure, for example:

• Ne smiješ pritiskati tipke. = You must not press buttons.
• Ne smiješ pritisnuti nijednu tipku. = You must not press any button.

So tu tipku strongly suggests a particular button.

Here are the basic dictionary forms:

• ne = not
• smiješ → dictionary form: smjeti = to be allowed, may
• pritisnuti = to press
• tu → dictionary base: taj / ta / to = that
• tipku → dictionary form: tipka = button, key

This is useful because in Croatian many words change form depending on person, gender, number, and case.