Learn Hardcore Swahili: Mama anajuta kununua simu isiyo janja; sasa anataka simu janja. - Mother regrets buying a non-smart phone; now she wants a smartphone.

In simu isiyo janja, isiyo is the negative relative marker that agrees with a class 9 noun (simu). It means that is not.
Formation: class‑9 agreement vowel i-
- negative -si-
  - relative ending -o → isiyo.
Use it before whatever completes the idea:
- simu isiyo janja = a phone that is not smart
- simu isiyo na kamera = a phone that doesn’t have a camera
- simu isiyofanya kazi = a phone that doesn’t work

Why can’t it just be simu si janja?

Simu si janja is a full sentence: the phone is not smart.
To use it as a modifier inside a noun phrase (the phone that is not smart), you need a relative construction: simu isiyo janja (or simu ambayo si janja).

Can I rewrite simu isiyo janja another way?

Yes. Common equivalents:

simu ambayo si janja (using the general relative pronoun)
simu isiyokuwa janja (literally, a phone that is not being smart; slightly heavier style) All are correct; isiyo is the most compact.

How would the negative relative change with other nouns?

It changes with the noun class. Examples:

Is janja an adjective that should agree with simu?

In the collocation simu janja, janja functions as part of a fixed phrase meaning smartphone. It doesn’t take the usual agreement prefix.
For ordinary adjectives you do see agreement: simu mpya (new phone), vitabu vipya (new books).
Note: mjanja is a different word (clever person). You wouldn’t say simu mjanja.

Is there a more formal or alternative term for smartphone?

Yes:

How do I say feature phone or non-smartphone more idiomatically?

Why is the infinitive kununua used after anajuta?

Swahili often uses the infinitive (ku- + verb) as a verbal noun after verbs like regret, start, like, prefer, etc.: anajuta kununua… = she regrets buying…
You can also use a finite clause: anajuta kwamba alinunua simu isiyo janja (she regrets that she bought…).

Can I use anajutia instead of anajuta?

Yes. Anajutia leans into the idea of regretting about/over something: Mama anajutia kununua simu isiyo janja.
Both juta and jutia are acceptable here; juta is very common with a following infinitive.

Should the second clause say anataka kununua simu janja, not just anataka simu janja?

Both are fine. Anataka simu janja = she wants a smartphone (as an object/possession).
Anataka kununua simu janja makes the action of buying explicit.

Where can I put sasa in the sentence?

Common, natural placements:

Sasa anataka simu janja.
Anataka simu janja sasa. Avoid inserting it awkwardly inside the noun phrase; keep it near the clause level.

What’s the nuance between anajuta and amejuta?

anajuta (present) suggests an ongoing state of regret.
amejuta (perfect) highlights the change-of-state as achieved or now relevant: she has come to regret it.
Both can fit; choice depends on the aspect you want.

How do I show definiteness (a/the/that) with simu janja when Swahili has no articles?

Use demonstratives or quantifiers:

Is the semicolon natural here?

Yes. Punctuation works much like in English. You could also use a period or a connector:

Could I use an object marker if I rewrote the buying part as a finite verb?

Yes, if the phone is already known/topical:

Alinunua simu isiyo janja. (no object marker; introducing it)
Aliinunua simu isiyo janja. (with object marker -i-, cross‑referencing a known phone) Avoid object-marking a brand-new, non-specific object.

Does simu janja change in the plural?

The form simu is both singular and plural (class 9/10). The adjective part stays the same: simu janja.
Agreement shows up elsewhere (e.g., verbs): Simu janja zimepanda bei. (Smartphones have gone up in price.)

Why is it just Mama and not mama yangu?

Mama can mean Mom as a proper name/title in context (hence capitalization), or simply mother/the lady depending on context.
To be explicit: Mama yangu anajuta… (my mother), or yule mama… (that lady).