Learn Hardcore Swahili: Ninapokosea sarufi, mwalimu anasahihisha kosa hilo. - When I make a grammar mistake, the teacher corrects that mistake.

Ninakosea sarufi. = I make a grammar mistake / I’m making a grammar mistake.
Ninapokosea sarufi, … = When(ever) I make a grammar mistake, …

Could I say Nikikosea sarufi, mwalimu anasahihisha kosa hilo instead of Ninapokosea sarufi? What is the difference between -po- and -ki-?

Yes, Nikikosea sarufi, mwalimu anasahihisha kosa hilo is grammatically correct, but there is a nuance:

-po- in ninapokosea focuses on time: when / whenever I make a mistake (a neutral, factual time clause).
-ki- in nikikosea often has a conditional feel: if / whenever I make a mistake (more like a condition).

Practically:

Ninapokosea sarufi… ≈ When(ever) I make a grammar mistake…
Nikikosea sarufi… ≈ If (or whenever) I make a grammar mistake…

In many everyday contexts they overlap, but -ki- is more obviously if, while -po- is more clearly when / whenever.

Where is the word “I” in this sentence? Why don’t we see mimi?

Swahili usually expresses the subject inside the verb using prefixes, rather than using an independent pronoun every time.

In ninapokosea, ni- already means I.
In anasahihisha, a- already means he / she.

So:

Ninapokosea sarufi… = When I make a grammar mistake…
Mwalimu anasahihisha… = The teacher corrects… (and we know mwalimu is the subject from word order).

You normally use mimi only for emphasis or contrast, e.g. Mimi ninapokosea… = When I (as opposed to someone else) make a mistake…

What is the difference between kosea and kosa here?

kosea is a verb: to be wrong / to make a mistake / to err.
- Ninapokosea sarufi = When I make a mistake in grammar.
kosa is a noun: a mistake / an error / a fault.
- kosa hilo = that mistake.

So the sentence uses both:

Ninapokosea sarufi – I make a mistake (verb) in grammar.
mwalimu anasahihisha kosa hilo – the teacher corrects that mistake (noun).

This is similar to English using both to err and an error, or to make a mistake and a mistake.

Why is there no word for “in” before sarufi? Why not ninapokosea katika sarufi?

In Swahili, many verbs naturally take a direct object where English would need a preposition.

kosea kitu = to be wrong in something / to make a mistake in something.

So:

Ninapokosea sarufi literally: When I-err grammar
but it is understood as When I make a mistake in grammar.

You can say ninapokosea katika sarufi, but it sounds heavier and less natural for this everyday sentence. The simple direct-object construction kosea sarufi is standard and idiomatic.

Why does the sentence say kosa hilo instead of just kosa?

kosa hilo literally means that mistake, referring back to the specific mistake already mentioned in the first clause.

First clause: Ninapokosea sarufi – I make a mistake in grammar.
Second clause: mwalimu anasahihisha kosa hilo – the teacher corrects that mistake (the one I just made).

Using only kosa here (anasahihisha kosa) would still be understandable, but kosa hilo is clearer and more natural because it ties the two clauses together and shows we are talking about the exact mistake just mentioned.

What is hilo doing after kosa? How does it agree with the noun?

Hilo is a demonstrative (that) agreeing with kosa in noun class.

kosa is a class 5 noun (pairs with class 6).
The class 5 “that” is hilo.
In Swahili, the common pattern for demonstratives is noun + demonstrative:
- kosa hilo = that mistake
- mtoto huyu = this child
- kiti kile = that chair (over there)

So kosa hilo shows:

noun class 5 (kosa), and
distal demonstrative hilo (that, not this).

Could we say hilo kosa instead of kosa hilo?

In standard, neutral Swahili, the usual position is:

noun + demonstrative: kosa hilo, mtu huyu, kitabu kile, etc.

You can place a demonstrative before the noun (hilo kosa) for strong emphasis or in certain stylistic/poetic contexts, but:

For a learner, kosa hilo is the natural, default word order.
hilo kosa sounds marked/emphatic and is much less common in everyday speech.

Why doesn’t anasahihisha have an object prefix, like analisahihisha kosa hilo?

Swahili can add an object prefix in the verb to point to the object:

Class 5 object prefix = li-
So: ana- (he/she) + li- (it, class 5) + -sahihisha (correct)
→ analisahihisha kosa hilo = the teacher corrects it, that mistake.

In practice:

You often see either:
- just the noun object: anasahihisha kosa hilo, or
- object prefix plus the noun for emphasis, clarity, or style: analisahihisha kosa hilo.

The sentence mwalimu anasahihisha kosa hilo is already perfectly natural; the object prefix is optional, not required.

What is the tense/aspect of ninapokosea and anasahihisha? Is this present, future, or habitual?

Both verbs use the present tense marker -na-, but the construction is effectively habitual / general:

Ninapokosea sarufi… – Whenever / when I make a grammar mistake…
…mwalimu anasahihisha kosa hilo. – the teacher corrects that mistake.

So the sentence describes a regular pattern or general rule, not a one-time event. English reflects this by using the simple present:
When I make a grammar mistake, the teacher corrects that mistake.

Where does the word mwalimu go in terms of grammar? Is it necessary if the verb already has a-?

Mwalimu is a full noun subject, and a- in anasahihisha is the subject prefix that agrees with it.

In Swahili, a finite verb almost always carries a subject prefix, even when the subject noun is present:

Mwalimu anasahihisha…
- mwalimu = the teacher
- a- = he/she (subject prefix agreeing with mwalimu)

Leaving out mwalimu and saying only Anasahihisha kosa hilo would still be grammatical, but then the subject would just be he/she, and you would not know it is the teacher unless context already makes that clear. The full noun mwalimu makes it explicit.