Learn Hardcore Swahili: Mpokezi anafungulia wageni mlango wa ofisi. - The receptionist opens the office door for the guests.

Mpokezi: receptionist; agent noun from -pokea (to receive), noun class 1.
a-: subject marker for class 1 (he/she).
-na-: present/imperfective marker (is/does).
-fungulia: verb stem meaning open for/to someone; from -fungua (open) + applicative suffix -lia (for/at/to).
wageni: guests/visitors; plural of mgeni (class 2).
mlango: door (class 3).
wa: of; possessive/associative linker agreeing with class 3 singular.
ofisi: office (class 9).

Why is it anafungulia and not anafungua?

Because -fungulia includes the applicative suffix -lia, which adds a beneficiary/goal: open something for/to someone.

anafungua mlango = opens a door (no beneficiary implied).
anafungulia wageni mlango = opens a door for the guests.

Do I need kwa to say “for the guests,” like anafungua mlango kwa wageni?

No. The applicative -lia already encodes “for.” You can use a periphrastic phrase for emphasis or formality:

anafungua mlango kwa ajili ya wageni = opens the door for the sake of the guests. But the most natural here is the applicative: anafungulia wageni mlango.

How would I say “The receptionist opens the office door for them”?

Use the object marker -wa- (them, class 2) before the verb stem:

Mpokezi anawafungulia mlango wa ofisi.

Is it okay to have both the object marker and the noun, like anawafungulia wageni?

Yes. Doubling (object marker + full noun) is common for emphasis or when the object is topical/definite:

Mpokezi anawafungulia wageni mlango wa ofisi.
Without doubling is also fine: Mpokezi anafungulia wageni mlango wa ofisi.

What is the most natural order of the two objects (wageni and mlango)?

With applicatives, the human/beneficiary usually comes before the inanimate theme:

Natural: anafungulia wageni mlango.
You can also hear anafungulia mlango wageni, but it’s less common unless there’s special emphasis or context. If you use the object marker for the beneficiary, you can drop the noun entirely:
anawafungulia mlango (wa ofisi).

What does -na- express here? Is it present progressive or simple present?

-na- is imperfective. It can be present progressive (is doing) or present habitual/generic (does), depending on context:

Right now: “is opening”
Generally: “opens”

How do I say this in past, perfect, and future?

Simple past: Mpokezi alifungulia wageni mlango wa ofisi. (opened)
Perfect/recent past: Mpokezi amefungulia wageni mlango wa ofisi. (has opened)
Future: Mpokezi atawafungulia wageni mlango wa ofisi. (will open)

How do I negate it?

Drop -na-, use the negative subject marker, and change the final vowel to -i:

No object marker: Mpokezi hafungulii wageni mlango wa ofisi.
With object marker: Mpokezi hawafungulii wageni mlango wa ofisi.

Does wageni mean “the guests” or “guests” in general? There’s no article.

Swahili has no articles. wageni can mean the guests or (some) guests; context decides. To make it clearly definite or specific:

wale wageni = those guests
baadhi ya wageni = some of the guests
wageni wote = all the guests

Why is it mlango wa ofisi and not mlango ya ofisi?

The possessive linker agrees with the first noun (mlango, class 3).

Class 3 singular takes wa: mlango wa ofisi If plural: milango (class 4) takes ya:
milango ya ofisi = the office doors

What’s the difference between ofisi and ofisini?

ofisi: the office (as a noun).
ofisini: at/in the office (locative).
So mlango wa ofisi = the office’s door; mlango wa ofisini = the door at the office (slight locative nuance).

Could I use kwa in the possessive, like mlango kwa ofisi?

No. Possession/association uses the class-agreeing linker (wa/ya/cha/la/za, etc.), not kwa. So use mlango wa ofisi, not mlango kwa ofisi.

What’s the passive voice version?

Two common options:

Beneficiary as subject: Wageni wanafunguliwa mlango wa ofisi (na mpokezi).
Theme as subject: Mlango wa ofisi unafunguliwa kwa wageni (na mpokezi).

What are the differences among -fungua, -funguka, -fungia, -fungulia?

-fungua: open (transitive) — you open something.
-funguka: open (intransitive) — something opens.
-fungia: shut/lock in, or close onto/with (applicative of -funga, to close).
-fungulia: open for/to/at (applicative of -fungua), adding a beneficiary/goal.

What exactly is mpokezi? Is there a difference from mapokezi?

mpokezi (class 1): a person who receives/greets; a receptionist/greeter.
mapokezi (class 6): reception (the reception area/service, not a person).
Example: mpokezi works at mapokezi.

Could I say “the guests’ office door” instead?

Yes, that’s a different meaning (an office that belongs to the guests):

mlango wa ofisi ya wageni (door of the office of the guests).
In the original sentence, mlango wa ofisi means the office door in general, not one owned by the guests.

If I switch the subject to plural, how does agreement change?

With a plural human subject (class 2), use wa- for the subject marker:

Wapokezi wanafungulia wageni mlango wa ofisi. (Receptionists open/are opening the office door for the guests.)
If you make the door plural too, adjust agreement in the possessive:
Wapokezi wanafungulia wageni milango ya ofisi.