Learn Hardcore Swahili: Tafadhali usiketi mbele ya mlango wa dharura. - Please do not sit in front of the emergency door.

So the structure is very close to English:
Please don’t sit in front of the door of emergency. → Please don’t sit in front of the emergency door.

Why is tafadhali at the beginning? Can it also go at the end of the sentence?

Tafadhali means please, and it’s flexible in position.

Beginning: Tafadhali usiketi mbele ya mlango wa dharura.
Middle (less common): Usiketi tafadhali mbele ya mlango wa dharura.
End: Usiketi mbele ya mlango wa dharura tafadhali.

All are grammatically fine. Putting tafadhali at the beginning or end is most natural.
The tone stays polite but clear that it is a request/prohibition, not just information.

What exactly is going on inside the word usiketi?

Usiketi is the negative form of a command (or subjunctive) addressed to you (singular).

From the verb kuketi – to sit (down):

keti! – sit! (positive command, 2nd person singular)
usi-keti → usiketi – don’t sit! (negative command, 2nd person singular)

Morphologically:

usi- – negative marker + you (singular)
keti – verb stem (sit)

So usiketi literally means you should not sit / do not sit.

Why is it usiketi and not something like usikete with an -e at the end?

In Swahili, the subjunctive/command form usually changes -a to -e:

ku-soma (to read) → soma! (read!) → usi-some (don’t read!)

But kuketi already ends in -i, not -a, and verbs ending in -i, -e, -u usually don’t change their final vowel in the subjunctive.

So:

kuketi → keti! → usiketi

No extra -e is added; the verb just stays keti.

Is usiketi singular or plural? How would I say “Don’t sit (you all) in front of the emergency door”?

Usiketi is singular: you (one person), don’t sit.

To address more than one person, Swahili uses a different subject marker:

msiketi mbele ya mlango wa dharura. – Don’t sit (you all) in front of the emergency door.

Breakdown:

m- – you (plural)
si- – negative
keti – sit

So:

Usiketi… – Don’t sit… (one person)
Msiketi… – Don’t sit… (several people)

What’s the difference between kiti, kuketi, and kukaa?

These are related but not the same:

kiti – chair
kuketi – to sit (down), especially taking a seat (often a bit more formal / standard)
kukaa – to sit / stay / live / remain, very common, broader meaning

You could also say:

Usikae mbele ya mlango wa dharura. – Don’t sit / don’t stay in front of the emergency door.

Meaning-wise, both usiketi and usikae work here.
Usiketi focuses on taking a seat there, usikae can also suggest don’t remain/occupy that spot.

What does mbele ya literally mean, and why do we need ya?

Mbele ya is a very common expression meaning in front of or before.

mbele – front, front side, ahead
ya – of (possessive/connector for certain noun classes)

Literally: mbele ya mlango = the front (part) of the door → in front of the door.

You normally need the ya after mbele when it connects to another noun:

mbele ya nyumba – in front of the house
mbele ya gari – in front of the car

Without ya, mbele just means front / ahead / forward by itself, not “in front of (something)”.
For example:

Nenda mbele. – Go forward / Go ahead.

Why is it mlango wa dharura and not mlango ya dharura?

The little word wa / ya / la / cha, etc. agrees with the first noun in a noun–noun phrase (what’s often called the of construction).

mlango (door) belongs to noun class 3/4 (m-/mi-).
For this class, the connector is wa in singular.

So:

mlango wa dharura – door of emergency → emergency door
milango ya dharura – doors of emergency → emergency doors

Notice how the connector changes with number:

singular: mlango wa dharura
plural: milango ya dharura

What exactly does dharura mean, and can I use it by itself?

Dharura means emergency, urgency, or urgent necessity.

As a noun:
- Kuna dharura. – There is an emergency.
- Ni hali ya dharura. – It’s an emergency situation.

In mlango wa dharura, it’s specifying what type of door it is: door of emergency → emergency door.

You’ll also see:

namba ya dharura – emergency number
huduma za dharura – emergency services

Is the word order fixed? Could I say Usiketi tafadhali mbele ya mlango wa dharura?

The core order is quite fixed:

Verb (with all its prefixes)
Prepositional phrase (mbele ya mlango wa dharura)

You can move tafadhali around because it’s a discourse/politeness word:

Tafadhali usiketi mbele ya mlango wa dharura.
Usiketi mbele ya mlango wa dharura tafadhali.
Usiketi tafadhali mbele ya mlango wa dharura.

All are grammatically correct. The meaning is unchanged; only emphasis or rhythm feels slightly different.
The most common are at the beginning or at the end.

How would I make this stronger, like a stricter warning instead of a polite request?

To sound stricter and less like a polite request, you can:

Drop tafadhali and keep the negative command:
- Usiketi mbele ya mlango wa dharura. – Don’t sit in front of the emergency door.
Or add emphasis with a word like kamwe (ever) or kabisa (at all):
- Usiketi kamwe mbele ya mlango wa dharura. – Never sit in front of the emergency door.
- Usiketi kabisa mbele ya mlango wa dharura. – Don’t sit in front of the emergency door at all.

Tone of voice also carries a lot of the “strictness” in real speech.

How would I say “Please don’t stand in front of the emergency door” instead?

Change only the verb from to sit to to stand:

tafadhali usisimame mbele ya mlango wa dharura.

Breakdown:

usi-simame – don’t stand (from kusimama, to stand)

Pattern to remember:

usi- + [verb] – don’t [verb] (you, singular)
msi- + [verb] – don’t [verb] (you all, plural)

How would I say it in a more neutral, informational way, instead of as a direct command?

You can turn it into a statement about what is allowed / not allowed:

Hakuruhusiwi kuketi mbele ya mlango wa dharura.
– You are not allowed to sit in front of the emergency door.

Or more general:

Hairuhusiwi kuketi mbele ya mlango wa dharura.
– It is not allowed to sit in front of the emergency door.

These sound like posted rules rather than a direct “Don’t do that” to someone.