Learn Hardcore Esperanto: Mia telefono devas funkcii en la oficejo.

In Esperanto, devi is a modal verb meaning “to have to / must.”
When you use a modal verb, the main action goes into the infinitive (the -i form):

Mi devas iri. – I have to go.
Ŝi volas dormi. – She wants to sleep.

So in your sentence:

Mia telefono devas funkcii.
- devas = has to / must
- funkcii = to function, to work

If you said Mia telefono funkcias en la oficejo, that would mean “My phone works in the office” (a simple statement of fact), without the idea of obligation or requirement.
With devas funkcii, you’re saying it is necessary that it work there.

What is the difference between devas and devus here?

devas = must / has to (real obligation, requirement, or strong expectation)
devus = should / ought to / would have to (more hypothetical, weaker, or more polite)

Compare:

Mia telefono devas funkcii en la oficejo.
My phone must work in the office. (It’s a requirement; maybe for my job.)
Mia telefono devus funkcii en la oficejo.
My phone should work in the office. (According to expectations, but maybe it doesn’t.)

Both are grammatically correct; they just express a different attitude about the obligation.

Why does funkcii end in -i and not -as, like funkcias?

Esperanto verbs have:

-as for present tense: funkcias = it works / is working
-is for past: funkciis
-os for future: funkcios
-us for conditional: funkcius
-i for the infinitive: funkcii = to work / to function

After devi (and other modal verbs like povi “can”, voli “want”), you use the infinitive:

Mi povas paroli. – I can speak.
Ŝi volas manĝi. – She wants to eat.
Mia telefono devas funkcii. – My phone has to work.

So funkcii is the infinitive “to function,” which is exactly what you need after devas.

Why is it Mia telefono, not Mian telefonon?

Because in this sentence the phone is the subject, not the object.

Mia telefono = my phone (subject form; no -n)
Mian telefonon = my phone as a direct object (with the accusative -n)

Your sentence structure is:

[Mia telefono] (subject)
[devas funkcii] (verb phrase)
[en la oficejo] (place where)

There is no direct object here: you’re not doing something to the phone; you’re saying what it has to do. So the noun stays in its basic form, telefono, without -n.

If you needed a direct object, then you’d add -n, e.g.:

Mi ŝaltas mian telefonon. – I turn on my phone.

What is the difference between mia and mi / min? Why not Mi telefono?

Esperanto clearly separates personal pronouns and possessive adjectives:

mi = I (subject)
min = me (object; mi
- -n)
mia = my
mian = my (used when the possessed noun is an object, e.g. mian telefonon)

Before a noun, you must use the possessive form:

mia telefono = my phone
(possessive adjective + noun)

You can’t say Mi telefono; that would be mixing “I” with “phone.” It has to be Mia telefono.

Why is it funkcii and not labori? Don’t both mean “to work”?

Both can translate as “to work,” but they’re used differently:

labori = to work (do labor, a job, effort)
Usually used for people or for the act of working:
- Mi laboras en banko. – I work in a bank.
funkcii = to function, to operate, to work (in the technical sense)
Used especially for machines, devices, systems:
- La komputilo ne funkcias. – The computer doesn’t work.
- Ĉi tiu ŝlosilo ne plu funkcias. – This key no longer works.

A phone is a device, so the natural verb is funkcii.
Mia telefono devas labori would sound odd, as if the phone is an employee doing a job.

Why is it en la oficejo and not ĉe la oficejo?

en = in, inside
- en la oficejo – inside the office (physically in the room/building)
ĉe = at, by, with (someone’s place / presence)
- ĉe la oficejo – at the office (in the area of the office, or by the office building)
- ĉe mia amiko – at my friend’s place

In your sentence you probably mean that the phone must work inside the office, not just somewhere near it, so en la oficejo is the most straightforward.

You can say ĉe la oficejo if you mean “in the vicinity of the office” or “at that location,” but en la oficejo is clearer for “indoors, in the office building/room.”

Why en la oficejo and not just en oficejo or en mia oficejo?

Esperanto la is the definite article (“the”).

en la oficejo = in the office (some specific office everyone knows from context)
en oficejo = in an office (some office, not specified which)
en mia oficejo = in my office

So:

Mia telefono devas funkcii en la oficejo.
Usually implies “in the office (where I work / that we have been talking about).”
Mia telefono devas funkcii en mia oficejo.
Emphasizes that it has to work in my personal office (as opposed to some other office).

All three are grammatically correct; you choose based on how specific you want to be.

Could it be en la oficejon with -n at the end of oficejo?

You add -n to a place when you mean movement toward it:

Mi iras en la oficejon. – I go into the office.
Ŝi kuras en la parkon. – She runs into the park.

In your sentence, you are talking about where the phone must work (its location), not about moving into that place. So you keep the simple form:

en la oficejo = in the office (state, location)
en la oficejon = into the office (direction, movement)

Thus Mia telefono devas funkcii en la oficejo is correct.

Can I change the word order, like En la oficejo mia telefono devas funkcii?

Yes. Esperanto word order is quite flexible. All of these are grammatically correct and natural, with only small differences in emphasis:

Mia telefono devas funkcii en la oficejo.
Neutral order; focus on the obligation.
En la oficejo mia telefono devas funkcii.
Slight emphasis on en la oficejo (“In the office, my phone has to work…”).
Mia telefono en la oficejo devas funkcii.
Emphasis can fall on devas funkcii after setting up “my phone in the office” as the topic.

You generally keep devas directly before funkcii, because they belong together as a verb phrase.

If I want to say “My phones have to work in the office,” how do I change the sentence?

You pluralize both the noun and the possessive:

Mia telefono devas funkcii en la oficejo.
My phone (singular) has to work…

becomes

Miaj telefonoj devas funkcii en la oficejo.
My phones (plural) have to work…

Details:

telefono → telefonoj (add -j for plural)
mia → miaj (adjective agrees with the noun’s number)

The rest (devas funkcii en la oficejo) stays the same.

Does telefono mean a landline, a mobile, or either? How would I say “cell phone”?

telefono in Esperanto is a general word for a telephone/phone, and can mean:

a landline
a mobile/cell phone
“the phone” in general, when context doesn’t matter

If you specifically want to say mobile / cell phone, some common expressions are:

poŝtelefono – literally “pocket-phone,” widely used for mobile phone
poŝtelefono inteligenta / inteligenta poŝtelefono – smartphone
poŝaparato or poŝaparato poŝtelefona (less common/older-style)

So you could also say:

Mia poŝtelefono devas funkcii en la oficejo. – My cell phone has to work in the office.

How should I pronounce Mia telefono devas funkcii en la oficejo?

Key points:

Stress is always on the second-to-last syllable of each word:
- MI-a
- te-LE-fo-no
- DE-vas
- FUNK-ci-i (three syllables: FUNK-ci-i)
- en
- la
- o-fi-CE-jo
Vowels are pure (like in Spanish or Italian):
- a as in “father”
- e as in “met”
- i as in “machine”
- o as in “more” (without sliding)
- u as in “food”
c is always like ts in “cats”:
- funkcii = foonk-TSI-ee
j is like English y in “yes”:
- oficejo = o-fi-TSE-yo

Putting it together (approximate English-friendly pronunciation):
MEE-ah teh-LEH-fo-no DEH-vas FOONK-tsee-ee en la o-fi-TSEH-yo.