Learn Hardcore Swahili: Mwalimu alituarifu kuhusu mtihani wa kesho. - The teacher informed us about tomorrow's exam.

a- = subject marker “he/she”
-li- = past tense “did”
-tu- = object marker “us”
-arifu = verb stem “inform” So alituarifu literally reads “he/she past us inform.”

How would I say “The teacher didn’t inform us about tomorrow’s exam”?

Use the negative past:

Mwalimu hakutuarifu kuhusu mtihani wa kesho. Breakdown: ha- (neg. 3sg) + -ku- (past in negatives) + -tu- (us) + -arifu (inform).

Why is it mtihani wa kesho and not something else? What does wa do?

wa is the associative “of” that agrees with the head noun’s class. Mtihani is class 3 (m-/mi-), whose singular associative is wa. Hence “exam of tomorrow” = mtihani wa kesho. In the plural it changes: mitihani ya kesho (“tomorrow’s exams”).

Could I say mtihani kesho instead of mtihani wa kesho?

Yes, but there’s a nuance:

mtihani wa kesho = “tomorrow’s exam” (possessive/associative, a bit more specific).
mtihani kesho = “the exam tomorrow” (adverbial feel, more like scheduling info). Both are natural.

What does kuhusu mean? Can I use juu ya instead, or omit the preposition?

kuhusu means “about/regarding” and is standard here.
juu ya is widely used in speech to mean “about,” though it literally means “on top of.” It’s acceptable in everyday usage: … alituarifu juu ya mtihani …, but many prefer kuhusu in careful/formal Swahili.
Don’t omit the preposition: alituarifu mtihani wa kesho sounds off. If you don’t use kuhusu/juu ya, switch to a “that”-clause: Mwalimu alituarifu kwamba/kuwa kutakuwa na mtihani kesho (“… informed us that there will be an exam tomorrow”).

Is Mwalimu “the teacher” or “a teacher”? Swahili has no articles, right?

Correct—no articles. Mwalimu can mean “a teacher” or “the teacher” depending on context. If you need to specify, add a demonstrative or quantifier:

yule mwalimu = that teacher
mwalimu mmoja = one/a certain teacher

Where do subject and object markers go in a Swahili verb?

Template: Subject marker + Tense/Aspect/Mood + Object marker + Verb stem (+ extensions) + Final vowel. In alituarifu: a- (SM) + -li- (past) + -tu- (OM) + arifu (stem).

Can I include a full pronoun with or instead of the object marker? For example, use sisi?

With OM only: Mwalimu alituarifu … (natural and common).
With a full pronoun (for emphasis): Mwalimu alituarifu sisi kuhusu … (both OM and pronoun).
Without OM: Mwalimu aliarifu sisi kuhusu … is also possible. Using both OM + noun/pronoun is very common when the object is human/definite and can add emphasis or clarity.

How would the sentence change if there were multiple teachers?

Pluralize the subject and adjust agreement:

Walimu walituarifu kuhusu mtihani wa kesho. Here wa- is the class 2 (people) subject marker, and -li- is still the past tense.

What are some common synonyms for kuarifu (“to inform”) and do they change the structure?

kuambia (to tell): Mwalimu alituambia kuhusu mtihani wa kesho.
kueleza (to explain): Mwalimu alitueleza kuhusu mtihani wa kesho.
kujulisha (to inform/let know): Mwalimu alitujulisha kuhusu mtihani wa kesho. Nuance: kuarifu/kujulisha sound a bit more formal/official; kuambia is the everyday “tell.”

Can I use the present perfect instead of the simple past? What’s the difference?

Yes:

Mwalimu ametuarifu kuhusu mtihani wa kesho. = “The teacher has informed us …” (recent/present relevance).
Mwalimu alituarifu … = simple past “informed us” (completed, narrative past).

How do I ask “What did the teacher inform us about?” in Swahili?

Mwalimu alituarifu kuhusu nini? You can also ask about a person: … kuhusu nani?

Why does wa agree with mtihani and not kesho?

In the associative construction X “of” Y, the connector agrees with the head noun (X). Here, the head noun is mtihani (class 3), so we use wa. kesho is the complement of that construction and does not control agreement.

Is there a more explicit way to say “The teacher informed us that the exam will take place tomorrow”?

Yes, use a “that”-clause and future on the exam:

Mwalimu alituarifu kwamba mtihani utafanyika kesho. Here uta- is the future marker agreeing with mtihani (class 3): u- (subject marker) + -ta- (future) + fanyika (take place).