Kabla sijasahau, naomba nikutakie heri ya mtihani wa kesho.

Literally, kabla sijasahau is:

• kabla – before
• si- – I (negative subject marker)
• -ja- – not yet / perfect aspect
• -sahau – forget

So sijasahau literally means I have not yet forgotten.

In Swahili, after kabla, it’s very common to use this negative perfect form to mean “before (something) happens.” So:

• kabla sijasahau = “before I have (yet) forgotten” → natural English: before I forget.

So the grammar is different, but the idea is the same: act now, at a time when the forgetting has not yet happened.

Both structures exist, but they’re used differently:

1. kabla sijasahau

   • Uses the negative perfect (si-
     • -ja-
       • verb).
   • Very standard and idiomatic after kabla to say “before I do X.”
   • Focuses on the state “I have not yet forgotten.”

2. kabla nisahau

   • Uses the subjunctive (ni-
     • -saha-u → nisahau).
   • Grammatically possible in some contexts, but kabla sijasahau is far more natural in everyday speech for “before I forget.”

So in normal conversation, kabla sijasahau is the preferred form.

Yes. Another common structure is:

• Kabla ya kusahau…

Here kusahau is the infinitive (to forget), and kabla ya + infinitive means “before doing X.”

So you might hear:

• Kabla sijasahau…
• Kabla ya kusahau…

Both are acceptable. Kabla sijasahau sounds a bit more idiomatic and personal (“before I (personally) forget”).

Literally, naomba means I request / I beg / I ask (for), from the verb -omba (to ask, request, beg, pray).

In modern polite Swahili, naomba… is often used like a softener, similar to please let me… or simply please….

Examples:

• Naomba niondoke sasa. – Please let me leave now.
• Naomba maji. – I’d like (some) water, please.

In your sentence:

• Naomba nikutakie heri…
  Literally: “I ask (that) I may wish you well…”
  Natural English: Let me wish you good luck… / I’d like to wish you good luck…

So it’s more than just the word “please”; it’s a polite, soft way of introducing what you want to do or ask.

Nikutakie breaks down like this:

• ni- – I (1st person singular subject prefix)
• -ku- – you (2nd person singular object prefix)
• -taki- – from the verb -takia (to wish [something] for someone)
• -e – subjunctive ending

So nikutakie literally means:

• “that I may wish you (something)” or “may I wish you…”

Because of the subjunctive -e, it often follows verbs like naomba (I request), nataka (I want), etc.:

• Naomba nikutakie heri. – I (politely) ask that I may wish you well.
• Nataka nikutakie heri. – I want to wish you well.

Both come from -takia (to wish [something] for someone), but:

1. nikutakie heri

   • ni- (I) + -ku- (you) + taki-
     • -e (subjunctive)
   • Often dependent on another verb: Naomba nikutakie heri…
   • Sense: “that I may wish you well / let me wish you well.”

2. nakutakia heri

   • na- (present tense marker) + ku- (you) + takia
   • Simple present: “I wish you well.”
   • Can stand alone: Ninakutakia heri ya mtihani. – I wish you good luck in the exam.

So your sentence uses a polite-subjunctive form wrapped in naomba; nakutakia would be a more direct statement.

Heri is a noun meaning things like:

• goodness, welfare, blessing, good fortune, best wishes.

In expressions of wishing someone well, it’s usually combined with ya to link it to a specific occasion or thing:

• heri ya mtihani – (good) wishes for the exam
• heri ya siku ya kuzaliwa – happy birthday (literally: blessings of the birthday)
• heri ya Krismasi – Merry Christmas

So heri ya… is a fixed pattern meaning good wishes / blessings for…

Break it down:

• heri – good fortune / blessing / best wishes
• ya – “of / for” (agreement with heri, which is class 9)
• mtihani – exam
• wa – “of” agreeing with mtihani (class 3/4)
• kesho – tomorrow

So:

• heri ya mtihani wa kesho
  = literally: “blessings/good wishes of the exam of tomorrow”
  = natural English: “good luck in tomorrow’s exam” or “best wishes for tomorrow’s exam.”

There are two “of” relationships:

• heri ya mtihani – wishes for the exam
• mtihani wa kesho – the exam of tomorrow (tomorrow’s exam)

The choice of wa / ya / cha / la / etc. is controlled by the noun class of the first noun in the relationship.

• mtihani (exam) is in noun class 3/4 (m-/mi-).
• The class 3 singular agreement for “of” is wa.

So:

• mtihani wa kesho – tomorrow’s exam
• mitihani ya kesho – tomorrow’s exams (mitihani is class 4, which takes ya)

Even though kesho itself doesn’t show class marking like normal nouns, the agreement still follows mtihani / mitihani.

Yes. Some natural variants include:

• Kabla sijasahau, nakutakia heri ya mtihani wa kesho.
  (Before I forget, I wish you good luck in tomorrow’s exam.)

• Nakutakia heri ya mtihani wa kesho.
  (I wish you good luck in tomorrow’s exam.) – without the “before I forget” part.

• Kabla ya kusahau, nakutakia heri ya mtihani wa kesho.
  Using kabla ya kusahau instead of kabla sijasahau.

Your original version is very natural, polite, and conversational.