Learn Hardcore Swahili: Usije ukasahau kufunga mlango kabla ya kulala. - Do not forget to lock the door before sleeping.

It literally means lest you come and then forget. Usije is the negative subjunctive of kuja (to come) used idiomatically to warn against an undesirable outcome, and ukasahau uses the consecutive -ka- to express the resulting action: and then you forget. Together: a strong caution—Don’t end up forgetting.

What exactly is the -ka- in ukasahau doing?

-ka- is the consecutive/narrative marker meaning and then (as a consequence of the previous clause). After a warning like usije, -ka- marks the possible result. Without -ka- here (usije usahau) sounds odd; usije ukasahau is the natural pairing.

Could I just say Usisahau kufunga mlango? What’s the difference?

Yes. Usisahau ... is a plain negative imperative: Don’t forget. Usije ukasahau ... is stronger and more cautionary—roughly Don’t go and forget / Make sure you don’t end up forgetting.

How do I say this to more than one person?

Use plural subject markers: Msije mkasahau kufunga mlango kabla ya kulala. The simpler reminder is Msisahau ... (Don’t forget, you all).

Where is the subject you in the sentence?

Swahili marks the subject on the verb. Here it’s u- (2nd person singular):

u-si-je = you + negative + subjunctive stem of come
u-ka-sahau = you + consecutive + forget For plural, use m-: m-si-je m-ka-sahau.

Why is it kufunga and not bare funga after sahau?

After verbs like sahau (forget), jaribu (try), anza (start), Swahili uses the infinitive ku- + verb. So kusahau kufunga = to forget to close. Bare funga would be an imperative.

Can I use an object marker and say kuufunga mlango or just kuufunga?

kuufunga mlango: grammatical but redundant; you’re repeating the object with both a pronoun and a noun. It’s used only for emphasis or as a resumptive if mlango was already established.
kuufunga: means to close it, with u- the object marker for noun class 3 (singular mlango). Use this when the door is understood from context.

Does kufunga mlango mean close or lock?

Primarily close/shut. Context can imply lock. To be explicit:

Close the door: funga mlango
Lock the door (with key/padlock): funga mlango kwa ufunguo/kufuli or simply funga kwa kufuli. Note: kufunga can also mean tie or fast (religious fasting), depending on the object.

Why is it kabla ya kulala and not just kabla kulala?

When kabla is followed by a noun or an infinitive (verbal noun), it takes ya: kabla ya X. The verb lala becomes a noun-like form with ku-: kulala. So kabla ya kulala = before sleeping / before you sleep.

Can I use the kabla huja- construction instead?

Yes: Kabla hujalala, usije ukasahau kufunga mlango. Here huja- is the not‑yet perfect (you have not yet ...) used after kabla to mean before you [do]. Both kabla ya kulala and kabla hujalala are idiomatic.

Can I front the time phrase?

Yes: Kabla ya kulala, usije ukasahau kufunga mlango. This is common and reads naturally.

How do I make this softer or more polite?

Add tafadhali (please): Tafadhali usije ukasahau kufunga mlango kabla ya kulala. A gentler alternative is Kumbuka kufunga mlango kabla ya kulala (Remember to close the door before sleeping).

Can ukasahau start a sentence by itself?

Generally no. -ka- links to a prior clause, so ukasahau expects something before it. As a standalone instruction, use usije ukasahau ... or the plain usisahau ....

Is a comma required after usije?

No. A slight pause is natural in speech, and a comma is optional for readability—especially if you front kabla ya ....

Can you break down every part of the sentence?

u-si-je = you + negative + subjunctive of come → a warning: lest you come (end up)
u-ka-sahau = you + consecutive and then + forget
ku-funga = to close/shut
m-lango = door (class 3; plural mi-lango)
kabla ya = before (of)
ku-lala = to sleep

If I wanted to report a past event instead of giving a warning, how would it look?

Use normal past tense: Ulisahau kufunga mlango kabla ya kulala. = You forgot to close the door before sleeping.

If I’m talking about multiple doors, what changes?

Use the plural milango (class 4) and, if you use an object marker, switch it to i-:

Usije ukasahau kufunga milango kabla ya kulala.
With an object marker referring back to milango: ... kuifunga (milango).