Tafadhali usiketi mbele ya mlango wa dharura.

Broken down word by word:

• Tafadhali – please
• usi-keti – don’t sit (negative command, 2nd person singular)
• mbele ya – in front of / before
• mlango – door
• wa dharura – of emergency

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.

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.

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.

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.

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)

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.

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.

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

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

The core order is quite fixed:

1. Verb (with all its prefixes)
2. 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.

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.

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)

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.