Learn Hardcore Swahili: Tangazo linaloonyesha bei ya chai limebandikwa ukutani. - The notice that shows the price of tea has been posted on the wall.

linaonyesha = “it shows / it is showing” (normal verb phrase, not relative)
linaloonyesha = “that shows / which is showing” (relative clause modifying a noun)

In this sentence, linaloonyesha bei ya chai describes tangazo (which notice? the one that shows the price of tea), so a relative form is needed, not a simple verb.

Compare:

Tangazo linaonyesha bei ya chai.
“The notice shows the price of tea.” (full sentence, main verb)
Tangazo linaloonyesha bei ya chai limebandikwa ukutani.
“The notice that shows the price of tea has been stuck on the wall.”
Here linaloonyesha… is an adjective‑like clause describing which notice.

Can I say Tangazo ambalo linaonyesha bei ya chai limebandikwa ukutani instead? What is the difference?

Yes, that is also correct.

linaloonyesha – “built‑in” relative on the verb (more compact, very common in spoken and written Swahili)
ambalo linaonyesha – uses the relative pronoun ambalo (agreeing with class 5 noun tangazo)

Nuance:

Tangazo linaloonyesha... – slightly more natural and flowing in many contexts.
Tangazo ambalo linaonyesha... – a bit more explicit/clear if you’re emphasizing the relative clause, or in some formal writing.

Semantically, there is no real difference; both mean “the notice that shows...”.

What noun class is tangazo, and how does that affect the sentence?

Tangazo belongs to noun class 5 (its plural is matangazo, class 6).

Class 5 agreement affects:

The relative verb:
- linaloonyesha
  li- (subject prefix, class 5)
  -lo- (relative marker, class 5)
The perfect/passive verb:
- limebandikwa
  li- (subject prefix, class 5)

If the noun were in a different class, these parts would change, e.g.:

kitabu kinachoonyesha... kimebandikwa... (class 7: kitabu “book”)
barua inayoonyesha... imebandikwa... (class 9: barua “letter”)

How is limebandikwa built, and what does its tense/aspect really mean?

Limebandikwa is a passive perfect form:

li- – subject prefix for class 5 (agreeing with tangazo)
-me- – perfect aspect marker (“has/have done”, resulting state)
bandik- – verb root (“stick, paste, affix”)
-w- – passive suffix
-a – final vowel

So limebandikwa = “has been stuck / has been posted / is (in the state of having been) stuck”.

The -me- form usually emphasizes a completed action with a present result, close to English “has been stuck” or sometimes simply “is stuck”.

Why is it bei ya chai and not just bei chai?

In Swahili, when one noun possesses or qualifies another, you normally use the “of” connector that agrees with the first noun.

bei (“price”) is class 9.
The class 9 “of” connector is ya.

So:

bei ya chai = “price of tea”

Without ya, bei chai would sound incomplete or wrong in standard Swahili. You nearly always put the appropriate -a connector between two nouns in this kind of relationship:

gari la baba – father’s car (class 5 gari → la)
kitabu cha Kiswahili – Swahili book (class 7 kitabu → cha)
bei ya sukari – price of sugar (class 9 bei → ya)

Why is it ukutani and not just ukuta?

Ukuta = “wall” (basic noun).
Ukutani = “on/at the wall” (with the locative suffix -ni).

The suffix -ni often indicates location:

nyumba → nyumbani – (at) home
meza → mezani – on/at the table
ukuta → ukutani – on/at the wall

In this sentence, limebandikwa ukutani means the notice has been stuck on the wall, so the locative -ni is appropriate.

Could I also say kwenye ukuta instead of ukutani? Is there a difference?

Yes, you can say:

Tangazo ... limebandikwa kwenye ukuta.

This is also correct and natural.

Difference:

ukutani – uses the locative suffix; often a bit shorter and very idiomatic.
kwenye ukuta – uses the preposition kwenye (“on, at, in”) + plain noun.

In many everyday contexts they are interchangeable, with only a slight stylistic difference. Ukutani can feel a bit more compact or “built‑in”; kwenye ukuta is slightly more explicit.

Does ya chai mean “of tea” in a possessive sense, or is it more like “tea-flavored/tea-related”?

Ya chai here is a straightforward “of” relationship:

bei ya chai = “the price of tea”

It doesn’t imply that tea “owns” something; it just links price and tea, like English “price of tea”.

This same structure is used for many relationships that English also expresses with of:

bei ya mkate – price of bread
bei ya maziwa – price of milk
kikombe cha chai – a cup of tea

Could we move parts of the sentence around, like starting with ukutani?

Yes, Swahili word order is somewhat flexible, especially with locations. For example:

Ukutani limebandikwa tangazo linaloonyesha bei ya chai.

This is still grammatical. The main changes:

ukutani is fronted for emphasis (“On the wall, there has been posted…”).
The verb limebandikwa still agrees with tangazo, which remains the logical subject.

The original order (Tangazo ... limebandikwa ukutani) is the most neutral and common, but fronting the location is used when you want to highlight where something is.

Could I say tangazo inayonyesha bei ya chai instead of linaloonyesha?

No, that would be grammatically wrong, because the agreement is with the wrong noun class.

inayoonyesha is for class 9/10 nouns (e.g. barua inayoonyesha... – a letter that shows...)
tangazo is class 5, so you must use the class 5 form linaloonyesha.

Relative and verb prefixes in Swahili must match the noun class of the noun they describe:

tangazo linaloonyesha... (class 5)
matangazo yanayoonyesha... (class 6 plural)
barua inayoonyesha... (class 9)
vitabu vinavyoonyesha... (class 8)

What is the nuance of tangazo in this sentence: does it mean “notice”, “advertisement”, or “announcement”?

Tangazo can mean all of these depending on context:

advertisement / ad
announcement
notice / posted sign

In this sentence, since it is stuck on the wall and shows the price of tea, “notice” or “sign” is usually the most natural translation into English: a posted price notice. But the core Swahili word tangazo covers all these related meanings.