Premu tiun butonon por malfermi la programon.

In Esperanto, -u is the verb ending for a command, request, or instruction.

So premi = to press, and premu = press!

In this kind of sentence, Esperanto uses the -u form where English often uses the bare verb:

• Premu = Press
• Atendu = Wait
• Rigardu = Look

It can sound like a direct command, but it is also very commonly used for neutral instructions, like on screens, manuals, and signs.

They both take -n because they belong to the direct object.

In Esperanto, the thing directly affected by the action usually gets -n:

• butono = a button
• butonon = a button / the button, as the direct object

Since tiun describes butonon, it also has to match it. This is called agreement.

So:

• tiu butono = that button
• tiun butonon = that button, as the object

Esperanto modifiers like tiu, adjectives, and participles usually match the noun in number and case.

Tiun is the accusative form of tiu, which means that.

So:

• tiu = that
• tiun = that, when it is the direct object

In this sentence, tiun butonon means that button.

If the sentence were the subject instead, you would use tiu:

• Tiu butono estas grava. = That button is important.

But here you are pressing the button, so it is the object:

• Premu tiun butonon.

Because por malfermi expresses purpose: in order to open.

• por = for / in order to
• malfermi = to open

So por malfermi la programon means to open the program or in order to open the program.

If you said malfermu, that would be another command:

• Premu tiun butonon kaj malfermu la programon. = Press that button and open the program.

That is different from saying the first action is done for the purpose of the second.

mal- is a very common Esperanto prefix meaning the opposite of.

So:

• fermi = to close
• malfermi = to open

This is one of the most characteristic features of Esperanto. Instead of learning a completely unrelated word, you often make an opposite with mal-:

• bona = good
• malbona = bad
• granda = big
• malgranda = small
• varma = warm
• malvarma = cold

So malfermi literally means something like to un-close.

Because it is the direct object of malfermi.

Inside the phrase por malfermi la programon, the verb is malfermi and the thing being opened is la programon, so it gets -n.

Compare:

• La programo malfermiĝas. = The program opens.
  Here programo is the subject, so no -n.
• Mi malfermas la programon. = I open the program.
  Here programon is the object, so it takes -n.

Even though the phrase is inside por malfermi..., the normal object marking still applies.

Esperanto uses la for the, but it does not always match English article usage exactly in every context.

Here, la programon probably refers to a specific program that both speaker and listener can identify, so la makes sense.

Meanwhile, tiun butonon already has tiun (that), and demonstratives like tiu already make the noun definite. Because of that, you do not use la together with tiu.

So:

• tiun butonon = that button
• not la tiun butonon

In Esperanto, words like tiu and ĉi tiu already do the job of specifying the noun.

Yes. Ĉi tiu means this, while tiu means that.

So:

• Premu tiun butonon = Press that button
• Premu ĉi tiun butonon = Press this button

Both are correct, but they point differently:

• tiu = that
• ĉi tiu = this

In practice, Esperanto often places ĉi after the word:

• tiu ĉi butono
• tiun ĉi butonon

That is very common and natural.

Esperanto word order is fairly flexible because the -n ending marks the object clearly.

So the normal order is:

• Premu tiun butonon por malfermi la programon.

But other orders can still be understood, for example:

• Por malfermi la programon, premu tiun butonon.

That version may sound a little more like To open the program, press that button.

Because tiun butonon and la programon both have -n, their grammatical roles stay clear even if the order changes. Still, the most natural style in instructions is usually straightforward and simple.

Esperanto’s -u form does not change for singular vs. plural.

So premu can mean:

• press! to one person
• press! to several people
• or a neutral instruction with no strong focus on who you is

Esperanto verbs do not change according to person or number:

• mi premas = I press
• vi premas = you press
• ili premas = they press

And the command form stays the same:

• Premu!

This is one reason Esperanto grammar is considered regular and simple.

Yes, in a broad sense por relates to purpose, so English can translate it in slightly different ways depending on context:

• to open the program
• in order to open the program
• for opening the program

In this sentence, the most natural English translation is usually to open the program or in order to open the program.

So the idea is: Press that button for the purpose of opening the program.

That is exactly how por + infinitive works in Esperanto.