Learn Hardcore Swahili: Kifaa kinahitaji umeme. - The device needs electricity.

It’s made of the subject concord for class 7 (ki-), the present tense marker (na-), and the verb stem hitaji (to need):
ki- + na- + hitaji → kinahitaji

Why is there no preposition before umeme?

In Swahili, the verb hitaji (to need) takes its object directly, without a preposition, so you say kinahitaji umeme.

What noun class is umeme, and how is it structured?

umeme (electricity) is class 3 (mass nouns) with the prefix u-, so the full noun umeme functions as the direct object here.

Where is the equivalent of “the” or “a” in this sentence?

Swahili has no separate articles; kifaa can mean “a device” or “the device” depending on context. To be more specific you can add a demonstrative (e.g., kifaa hicho “that device”).

How would you make the sentence plural (“devices need electricity”)?

Change kifaa (class 7) to its plural vifaa (class 8) and adjust the verb concord to vi-:
Vifaa vinahitaji umeme.

How do you turn it into a question (“Does the device need electricity?”)?

Use the interrogative particle je:
Je kifaa kinahitaji umeme?
Or simply rely on intonation:
Kifaa kinahitaji umeme?

Can you omit the noun kifaa and just say kinahitaji umeme?

Yes. The subject concord ki- on the verb carries the meaning “it,” so Kinahitaji umeme is grammatically correct but may be less clear without context.