Learn Hardcore Swahili: Weka blanketi juu ya kitanda kwa sababu usiku unakuwa baridi. - Put a blanket on the bed because the night gets cold.

weka = put/place (neutral, most common).
wekelea = set/place gently; also fine: wekelea blanketi juu ya kitanda.
funika = cover. If you mean “cover the bed with a blanket,” use funika kitanda kwa blanketi.

What is the plural of blanketi, and what noun class is it?

blanketi is a loanword in noun class 5/6.
Singular: blanketi. Plural: mablanketi.

Why is it juu ya kitanda and not juu la kitanda or cha kitanda?

juu is a locative noun that takes the genitive connector ya, so you get juu ya “top of.”
la/cha agree with different head nouns. Here the head is juu, so ya is required. (Example with a different head: kifuniko cha kitanda “bed cover.”)

What’s the difference between juu ya kitanda, kwenye kitanda, and kitandani?

juu ya kitanda = on top of the bed (explicitly the upper surface).
kwenye kitanda = on/at the bed (general location).
kitandani (-ni locative) = on the bed / in bed (short, idiomatic). You can say: Weka blanketi kitandani.

What noun class is kitanda, and what’s its plural?

Class 7/8 (ki-/vi-). Singular: kitanda. Plural: vitanda.

Does usiku need a preposition to mean “at night”?

No. Usiku can function adverbially by itself. In this sentence it’s the subject (usiku unakuwa baridi), but you can also say Ninafanya kazi usiku “I work at night.”

Why is it unakuwa and not inakuwa or something else?

Agreement. Usiku is in the U-classes (11/14), whose subject prefix is u-. Hence u-na-kuwa → unakuwa.

inakuwa would agree with class 9/10 nouns.
anakuwa would agree with a class 1 noun (he/she).

What exactly does unakuwa baridi mean here?

From kuwa “to be/become.” With -na- it means “is becoming/gets.” So usiku unakuwa baridi = “at night it gets/becomes cold.”
For a general tendency you can use the habitual: Usiku huwa baridi “Nights are usually cold.”

Could I say kwa sababu usiku ni baridi instead?

Yes.

ni baridi = “is cold” (simple state/general fact).
unakuwa baridi = “gets/becomes cold” (emphasizes change).
Choose based on the nuance you want.

How do I say “because of the cold at night” with a noun phrase?

Use kwa sababu ya + noun:

Weka blanketi juu ya kitanda kwa sababu ya baridi usiku.
Compare the clausal version: … kwa sababu usiku huwa/unakuwa baridi.

Where can I place the kwa sababu clause?

Either before or after the main clause:

Kwa sababu usiku unakuwa baridi, weka blanketi juu ya kitanda.
Weka blanketi juu ya kitanda kwa sababu usiku unakuwa baridi.
Use a comma when the reason clause comes first.

Do I need an object marker for blanketi with weka?

No, not when the object is stated after the verb. Object markers replace a known object.
Example with an object marker (class 5 singular = -li-):

Ninaliweka juu ya kitanda. = “I’m putting it on the bed.”

Is baridi an adjective?

It’s primarily a noun (“cold, coldness”), but it’s used predicatively: (kitu) ni baridi / kinakuwa baridi = “(something) is/gets cold.” You can also say kuna baridi = “it’s cold/there is cold.”

Any pronunciation tips for juu and kwa sababu?

juu has a long vowel: say “joo” (hold the u).
kwa sababu is three words; in fast speech they may run together, but write them separately.
Penultimate stress: we-KA, blan-KE-ti, ki-TAN-da, sa-BA-bu, u-NA-KU-wa, ba-RI-di.

How do I address one person vs a group without sounding rude?

One person: Weka blanketi …
Group: Wekeni blanketi …
Add politeness as needed: Tafadhali weka/wekeni … or Naomba uweke/muweke ….