Learn Hardcore Swahili: Leo mimi ninakutakia afya njema.

ni- is the required grammatical subject prefix on the verb for “I”. You can’t drop it.
mimi is the independent pronoun “I / me”. It is optional and mainly adds:
- emphasis: I (as opposed to someone else)
- clarity in context
- a more careful/polite tone in some situations

Without mimi, the sentence is still complete:

Leo ninakutakia afya njema. – Today I wish you good health.

With mimi, you’re stressing I:

Leo mimi ninakutakia afya njema. – Today I (myself) wish you good health.

How is the verb ninakutakia formed, and what does each piece do?

ninakutakia is made of several parts:

ni- – subject prefix for I
-na- – present tense marker
-ku- – object marker for you (singular)
-takia – verb root “to wish (for someone)”

So:

ni-na-ku-takia → I-present-you-wish → I am wishing you / I wish you

The verb -takia is related to -taka (to want), but in practice kutakia (mtu jambo) is understood as “to wish someone something”:

Ninakutakia heri. – I wish you well.
Ninawataikia mafanikio. – I wish you (pl.) success.

What exactly is the -ku- in ninakutakia? Is it the infinitive “ku-” or something else?

Here -ku- is not the infinitive marker. It is the object marker for “you (singular)”.

Infinitive: ku- at the beginning of a verb:
- kutaka – to want
- kutakia – to wish (someone)
Object marker: -ku- inside a conjugated verb, between tense and the verb root:
- ni-na-ku-takia – I (am) you-wishing
- nakupenda – I love you (ni-na-ku-penda → nakupenda)

So in ninakutakia, -ku- = you, not to.

Is it necessary to include mimi? Can I just say Leo ninakutakia afya njema?

You can and very often will omit mimi.

Leo ninakutakia afya njema. – Fully correct, natural.
Leo mimi ninakutakia afya njema. – Also correct, but with a bit more emphasis on I.

mimi is mainly for emphasis, contrast, or politeness. Grammatically, the verb ni-na- already tells us the subject is “I”.

Can I shorten ninakutakia to nakutakia? Is that normal?

Yes. In everyday Swahili, ni-na- usually contracts to na- in the present tense:

ninasoma → nasoma – I am reading / I read
ninakula → nakula – I am eating / I eat
ninakutakia → nakutakia

So you can say:

Leo nakutakia afya njema. – very common, natural spoken and written form.

The meaning is the same; nakutakia still means “I (present) you-wish”. The ni- is “hidden” inside that na- in standard usage.

Can the word leo go in a different position, or must it be at the beginning?

Leo is flexible. All of these are grammatical; the difference is only in emphasis or style:

Leo ninakutakia afya njema. – Today I wish you good health.
Ninakutakia afya njema leo. – I wish you good health today.
Leo mimi ninakutakia afya njema. – Today I (myself) wish you good health.
Mimi ninakutakia leo afya njema. – I wish you today good health (less common ordering, but possible).

Position of leo changes focus slightly but doesn’t change the basic meaning. Sentence-initial Leo is probably the most typical.

What’s the difference between afya njema and afya nzuri?

Both mean “good health” and are correct.

afya njema
- njema = good, fine, wholesome, often slightly formal or “nice” in tone
- Often used in wishes and greetings:
  - Nakutakia afya njema. – I wish you good health.
afya nzuri
- nzuri = good, nice, beautiful (very common adjective)
- Also fine, but afya njema is more idiomatic in set expressions about health.

You can use either, but afya njema sounds particularly natural in this kind of well‑wishing phrase.

How would I say “I wish you (plural) good health” instead of singular “you”?

Change the object marker from -ku- (you singular) to -wa- (them/you plural, for people):

Leo ninawatakia afya njema.
- ni- – I
- -na- – present tense
- -wa- – you (plural) / them
- -takia – to wish (for)

So:

Leo ninakutakia afya njema. – Today I wish you (one person) good health.
Leo ninawatakia afya njema. – Today I wish you (several people) good health.

Could I say Leo mimi ninakutakia wewe afya njema, adding wewe after the verb?

Normally, you don’t say it that way, because -ku- already marks the object “you”. In Swahili you usually choose either:

object marker on the verb (-ku-)
or
an independent pronoun/noun phrase after the verb

But not both, unless you really want to emphasize or clarify:

Leo ninakutakia afya njema. – Normal, natural.
Leo ninakutakia wewe afya njema. – Possible, but sounds heavy/over‑explicit; would be used only for strong emphasis on you specifically.

For everyday speech, stick to Leo ninakutakia afya njema (no extra wewe).

How would I change this sentence to past or future tense?

Change the tense marker in the verb:

Present (given)
- Leo ninakutakia afya njema. – Today I wish you good health.
- Verb: ni-na-ku-takia
Past
- Leo nilikutakia afya njema. – Today I wished you good health.
- Verb: ni-li-ku-takia
  - -li- = past tense marker
Future
- Kesho nitakutakia afya njema. – Tomorrow I will wish you good health.
- Verb: ni-ta-ku-takia
  - -ta- = future tense marker

So the pattern is:
ni-[tense]-ku-takia for I [tense] wish you.

Is Leo mimi ninakutakia afya njema a common thing to say in real life? In what context would it be used?

Yes, it’s natural, though you’ll hear it more in slightly formal or extended well‑wishing contexts, for example:

in a card, letter, email, or SMS to someone who is ill or going away
in holiday or New Year messages (with other words added)
when speaking kindly to someone dealing with health issues

In casual spoken Swahili, you might hear simpler variations:

Nakutakia afya njema. – I wish you good health.
Nakutakia heri ya afya. – I wish you the best of health.

The sentence you gave is fully correct and polite; it just leans a bit toward careful or thoughtful speech rather than quick small talk.