Learn Hardcore Swahili: Tafadhali, usiseme kwa sauti kubwa; mtoto analala chumbani. - Please, do not speak loudly; the child is sleeping in the room.

In Swahili, tafadhali can appear at the beginning of the sentence, as it does here, but you may also hear it in other positions depending on style and emphasis. Starting with it is very natural for polite requests.

Why is it usiseme and not sema?

Usiseme is the negative singular command form of -sema (to say / to speak).

sema = speak! / say!
usiseme = don’t speak! / don’t say!

This is a very important Swahili pattern:

u- = negative marker for a singular command
-si- = part of the negative structure
-seme = verb stem in the form used after negation

So usiseme means don’t speak when talking to one person.

If you were talking to more than one person, you would say:

msiseme = don’t speak (plural)

Does -sema mean say or speak here?