Learn Hardcore Swahili: Bomba likiziba, tafadhali funga maji na mpigie fundi simu. - If the pipe clogs, please turn off the water and call the repairman.

It’s the “when/if” linker placed inside the verb. The breakdown is: li- (class 5 subject for bomba) + -ki- (“when/if/whenever”) + ziba (“block/clog”). So Bomba likiziba = “When/if the tap/pipe clogs (up).”
More examples:

Gari likikwama, niite. = If/when the car gets stuck, call me.
Mlango ukifunguka, ingia. = When/if the door opens, come in.

Why is the subject prefix li- used with bomba?

Because bomba is in noun class 5 (ji-/Ø), whose subject prefix is li- in the present/habitual. For instance: bomba linavuja (the pipe is leaking), tunda linaiva (the fruit is ripening). If it were plural (class 6), the subject prefix would be ya-: mabomba yanavuja (the pipes are leaking).

Could I say Kama bomba limeziba instead of Bomba likiziba? What’s the difference?

Yes.

Bomba likiziba uses the -ki- form, meaning “when/if (it) clogs,” often general or prospective.
Kama bomba limeziba uses the conditional word kama
- perfect -me- (“has”), meaning “if the tap has (already) clogged.” It sounds a bit more like a specific situation that has already happened.

Is ziba the right verb for “get blocked,” or should it be passive -zibwa?

In everyday Swahili, ziba can mean “to block/plug” and also “to get blocked” intransitively: bomba linaziba (the pipe gets blocked). The passive -zibwa (“be blocked by [something]”) is also correct: Bomba likizibwa = “When the pipe is (being) blocked.” Your sentence’s likiziba is natural and common.

What does tafadhali do here, and where can it go?

Tafadhali means “please.” It can come at the start, middle, or end of a request:

Tafadhali funga maji.
Funga maji, tafadhali.
Tafadhali, funga maji na mpigie fundi simu. All are polite; initial placement is very common.

Why funga maji and not zima maji?

funga = close, fasten, shut. With water/gas valves and taps, Swahili uses funga: funga maji (turn off/shut the water), fungua maji (turn on/open the water).
zima = turn off/extinguish (lights, fire, devices): zima taa, zima moto, zima friji.
So for taps/valves, funga/fungua is the idiomatic pair.

What exactly is mpigie in mpigie fundi simu?

m- = object marker “him/her” (class 1 person)
pigie = subjunctive of pigia (“call to, hit for, do for” – the applicative of piga)
Together: mpigie = “(you should) call him/her.” With fundi specified, it means “call the technician.”

Why is it pigie (with -e) and not pigia?

Because it’s the subjunctive form, which ends in -e and is the standard way to give polite commands or requests, especially when you need to host an object marker in front: m-pigie. A bare imperative without an object marker would be pigia (Pigia fundi simu!), but adding m- typically shifts you to the subjunctive: mpigie (fundi simu).

Do I have to include both the object marker m- and the noun fundi?

Not strictly, but it’s very common with humans/definites. Options:

Mpigie fundi simu. (with OM + noun; very natural)
Pigia fundi simu. (no OM; also fine)
Mpigie simu. (OM only; relies on context to know who) Including both often feels more definite/emphatic: “call the technician (that one we’re talking about).”

Why add simu after mpigie fundi?

Because piga simu is the idiom for “make a phone call.” Pigia/ mpigie (mtu) simu = “call (someone) on the phone.” Without simu, piga/pigia can mean “hit/strike/do for,” so simu clarifies you mean a phone call.

Is fundi specifically a plumber here?

Fundi means a craftsperson/technician in general. Context decides the specialty. If you want to be specific, say:

fundi bomba / fundi wa mabomba = plumber
fundi umeme = electrician
fundi simu = phone technician

Could I say Bomba likiziba, tafadhali funga bomba instead of funga maji?

Yes, funga bomba (close the tap) is idiomatic too.

funga maji focuses on shutting the water supply.
funga bomba focuses on closing the tap itself.
Both are natural in this context.

What if I’m talking to more than one person?

Use the plural imperatives:

tafadhali fungeni maji (please turn off the water)
mpigieni fundi simu (call the technician) So: Bomba likiziba, tafadhali fungeni maji na mpigieni fundi simu.

How would I make it negative, like “don’t turn off the water”?

Use the negative imperative with the subjunctive:

Singular: Usifunge maji. (Don’t turn off the water.)
Plural: Msifunge maji. With the call: Usimpigie fundi simu / Msimpigie fundi simu (don’t call the technician).

Does na here mean “and,” or could it mean “then”?

Na primarily means “and.” In instructions, it often feels like “and then” in English. If you want to be explicitly sequential, you can use kisha or halafu:
Bomba likiziba, tafadhali funga maji kisha mpigie fundi simu.

Is bomba always “tap,” or can it be “pipe”?

It’s both. Bomba commonly refers to a pipe and also to a faucet/tap (especially in East Africa). If you need to be specific:

bomba la maji (water pipe/tap; context decides)
mfereji is also used for “tap/faucet” in some areas, but it can mean “channel/canal” too. Context helps.