Learn Hardcore Swahili: Shati la mikono mirefu linanilinda dhidi ya baridi usiku. - The long-sleeved shirt protects me against the cold at night.

Shati is in noun class 5 (ji-/Ø, plural ma-), and the class‑5 genitive is la.
So shati la … literally means “shirt of …” → “a shirt with …”

If the head noun were in a different class, the genitive would change, for example:

nguo ya mikono mirefu – a long‑sleeved garment (nguo is class 9 → ya)
viatu vya ngozi – shoes of leather (viatu class 8 → vya)

In shati la mikono mirefu, la agrees with shati, not with mikono. The rule is: genitive marker agrees with the first noun (the thing being described), not with the following noun(s).

Why does mikono (which I thought means “hands”) refer to sleeves here?

In Swahili, mkono (plural mikono) can mean:

hand
arm
sleeve

The idea is “the part that sticks out from the body/clothing.” So:

mkono – hand/arm/sleeve (singular)
mikono – hands/arms/sleeves (plural)

In clothing contexts, mikono almost always means sleeves.
So shati la mikono mirefu = a shirt of long sleeves → a long‑sleeved shirt.

Why is it mirefu and not mrefu in mikono mirefu?

Adjectives in Swahili agree with the noun class and number of the noun they describe.

Root of the adjective: -refu (long, tall)
Singular class 3 (mkono) uses mrefu
Plural class 4 (mikono) uses mirefu

So:

mkono mrefu – a long arm/hand/sleeve (singular)
mikono mirefu – long arms/hands/sleeves (plural)

In the sentence we have mikono (plural), so we must say mirefu, not mrefu.

How is linanilinda formed, and what does each part mean?

Linanilinda breaks down like this:

li- – subject prefix for class 5 (here referring back to shati)
-na- – present tense marker (general present / present continuous)
-ni- – object marker “me”
-linda – verb root “protect”

So:

linanilinda = “it (class 5) is protecting me” / “it protects me”

Literally: li‑na‑ni‑linda → “it‑PRES‑me‑protect”.

Could I say shati lenye mikono mirefu instead of shati la mikono mirefu?

Yes, you can, and it’s very natural:

shati la mikono mirefu – a shirt of long sleeves
shati lenye mikono mirefu – a shirt having long sleeves

-enye is an adjective stem meaning “having/with”, and it also agrees with noun class:

shati lenye … (class 5)
vitabu vyenye … (class 8)
nyumba yenye … (class 9)

Both versions are correct. In everyday speech, shati la mikono mirefu is very common; shati lenye mikono mirefu feels a bit more “descriptive” or explicit: “a shirt that has long sleeves.”

Why is the subject prefix li- used in linanilinda for shati?

In Swahili, verbs take a subject prefix that agrees with the noun class of the subject.

shati is in noun class 5.
The class‑5 subject prefix in the present tense is li-.

Some examples:

shati linanilinda – the shirt protects me.
tunda linaiva – the fruit is ripening.
gari linakuja – the car is coming.

So li- in linanilinda is simply agreeing with shati in class 5.

What is the nuance of dhidi ya baridi? Could I say kutoka kwa baridi or just baridi?

Dhidi ya means “against” (in opposition to), and it is a fixed prepositional phrase:

dhidi ya baridi – against the cold
kunilinda dhidi ya baridi – to protect me against the cold

Alternatives and their feel:

kunilinda dhidi ya baridi – “protect me against cold” (very standard, clear).
kunilinda kutoka kwa baridi – literally “protect me from cold”; understandable, but dhidi ya baridi is the idiomatic set phrase here.
Just kunilinda baridi is not correct; you need a preposition (like dhidi ya).

In short: dhidi ya baridi is the natural, idiomatic way to say “against the cold” in this kind of sentence.

Is baridi here a noun or an adjective, and why doesn’t it have any prefix?

In this sentence, baridi functions as a noun meaning “cold, coldness”.

Many class 9/10 nouns in Swahili have no visible noun prefix; baridi is one of them.
As a noun, it can take agreements: baridi kali, baridi hii, etc.

It can also be used adjectivally (“cold” as a property):

maji baridi – cold water
chakula baridi – cold food

In dhidi ya baridi, it is being treated as a general, abstract noun: “the cold.” No extra prefix is needed because that’s how class 9 nouns work.

Why is usiku just placed at the end? Is that the standard way to say “at night”?

Yes, usiku on its own commonly means “at night / in the night” when used as a time expression. Swahili often puts time expressions at the end (or also at the beginning) of the sentence without extra prepositions:

Ninafanya kazi mchana. – I work in the daytime.
Tulifika asubuhi. – We arrived in the morning.
Shati … linanilinda dhidi ya baridi usiku. – The shirt protects me against the cold at night.

You could also say:

linanilinda dhidi ya baridi ya usiku – against the cold of the night (more specific, a bit more formal or descriptive).

But baridi usiku (cold at night) is perfectly natural and common in speech.

Could I move usiku to the beginning or somewhere else in the sentence?

Yes. Time expressions like usiku are quite flexible in position. All of these are grammatical, with only slight changes in emphasis:

Usiku, shati la mikono mirefu linanilinda dhidi ya baridi.
– At night, my long‑sleeved shirt protects me against the cold.
(Emphasis on the time.)
Shati la mikono mirefu usiku linanilinda dhidi ya baridi.
– Still understandable; here usiku is inserted after the noun phrase. It’s less common than putting usiku at the very end, but context can make it clear.
Shati la mikono mirefu linanilinda dhidi ya baridi usiku.
– The most neutral and natural: “…against the cold at night.”

So the textbook‑style, most natural version is the original, with usiku at the end.