Malfermu la retumilon.

Because -u is the Esperanto ending for the volitive / imperative form.

So malfermu means something like:

• open!
• please open
• let ... open, depending on context

In this sentence, it is a direct command or instruction: Open the browser.

A few related forms of the same verb are:

• malfermi = to open
• malfermas = opens / is opening
• malfermis = opened
• malfermos = will open
• malfermu = open! / please open

Yes. malfermu comes from:

• fermi = to close
• mal- = the opposite of
• malfermi = to open

So Esperanto often makes opposites with mal-.

Examples:

• bona = good → malbona = bad
• granda = big → malgranda = small
• fermi = close → malfermi = open

Then malfermu is the command form of malfermi.

There are two different endings here doing two different jobs:

• la = the
• -n on retumilon = marks the direct object

So:

• retumilo = browser
• la retumilo = the browser
• la retumilon = the browser, as the object of the verb

Since the sentence is telling someone to open something, the browser is the thing being opened, so Esperanto marks it with -n.

Because it is the direct object of the verb.

Esperanto usually adds -n to the noun that receives the action.

Here, the action is opening, and the thing being opened is the browser:

• Malfermu la retumilon. = Open the browser.

Compare:

• Mi malfermas la retumilon. = I open the browser.
• Mi fermas la pordon. = I close the door.
• Legu la libron. = Read the book.

In all of those, retumilon / pordon / libron are direct objects, so they take -n.

It is a built word:

• reto = net, network
• retumi = to browse the internet / surf the web
• -ilo = tool, instrument

So retumilo is literally something like a tool for web-browsing — in normal English, browser.

The suffix -ilo is very common in Esperanto:

• tranĉi = to cut → tranĉilo = knife
• skribi = to write → skribilo = writing tool, pen
• komputi = to compute → komputilo = computer

Yes, in normal modern Esperanto, retumilo usually means a web browser such as Chrome, Firefox, Safari, and so on.

That is because it comes from retumi, which is connected with browsing the internet/web.

So in a computer instruction like this, retumilo is definitely understood as browser.

It is understood, not stated.

Esperanto commands often leave out the subject, just like English does:

• Open the browser.
• Malfermu la retumilon.

If you want, you can make the subject explicit:

• Vi malfermu la retumilon. = You, open the browser.

But that is usually unnecessary unless you want emphasis or contrast.

By itself, Malfermu la retumilon is a plain command or instruction. Whether it feels rude, neutral, or polite depends a lot on context and tone.

In instructions, manuals, or software help, this kind of command is completely normal.

If you want to sound more explicitly polite, you could say:

• Bonvolu malfermi la retumilon. = Please open the browser.
• Mi petas, malfermu la retumilon. = I ask/request: open the browser.

So malfermu is not automatically rude; it is just the normal imperative form.

You probably could, but it would usually sound less natural if you mean a specific browser.

• Malfermu la retumilon. = Open the browser.
• Malfermu retumilon. = Open a browser / open browser, more indefinite

In many practical contexts, especially computer instructions, la is natural because both speaker and listener know which browser is meant.

Esperanto does not have an indefinite article like English a/an, so a bare noun can sometimes feel more general or indefinite.

This is the most straightforward word order:

• verb first: Malfermu
• object after it: la retumilon

Because Esperanto marks the object with -n, word order is more flexible than in English. So other orders are possible, such as:

• La retumilon malfermu.

That can add emphasis, but the normal neutral order for a command is:

• Malfermu la retumilon.

A simple pronunciation guide is:

mahl-FEHR-moo lah reh-too-mee-LOHN

A few helpful points:

• a sounds like a in father
• e sounds like e in bet, but often a bit clearer
• i sounds like ee
• o sounds like o in for or a pure o
• u sounds like oo
• stress is on the second-to-last syllable

So:

• mal-FER-mu
• re-tu-mi-LON

It is singular.

In Esperanto:

• -o = singular noun
• -oj = plural noun
• -n = accusative ending
• -ojn = plural + accusative

So:

• retumilo = a browser
• retumiloj = browsers
• retumilon = a browser / the browser as object
• retumilojn = browsers / the browsers as object

Here, retumilon means one browser being opened.

Yes. Malfermi is used both literally and digitally, much like English open.

Examples:

• malfermi pordon = open a door
• malfermi libron = open a book
• malfermi dosieron = open a file
• malfermi retumilon = open a browser

So this sentence sounds perfectly natural in a computer context.