Usiweke mayonesi nyingi kwenye saladi ya leo.

Because Swahili negative commands normally use the subjunctive ending -e.

So from kuweka (to put), you get:

• Weka! = Put!
• Usiweke! = Don’t put!

For a learner, the important pattern is:

• usi- + verb stem + -e for a negative command to one person

So usiweka would sound wrong here.

It is talking to one person.

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

• Msiweke mayonesi nyingi kwenye saladi ya leo.

So the contrast is:

• usiweke = don’t put (singular)
• msiweke = don’t put (plural)

This is because Swahili adjectives have to agree with the noun class of the noun they describe.

In this sentence, mayonesi is being treated with the agreement pattern that gives nyingi. That is why you get:

• mayonesi nyingi

and not forms like mayonesi mengi or mayonesi mingi in this sentence.

This kind of agreement is very common with loanwords in Swahili.

Here it means much / a lot of, because mayonesi is a mass noun in this context, like water or sugar in English.

So:

• mayonesi nyingi = a lot of mayonnaise / much mayonnaise

Even though -ingi is often translated as many in basic vocabulary lists, with mass nouns it is often best understood as much or a lot of.

Good question. Literally, the Swahili is closer to:

• Don’t put a lot of mayonnaise in today’s salad.

In natural English, that is often expressed as:

• Don’t put too much mayonnaise...

So the sense of excess comes from the negative instruction plus nyingi (a lot / much).

If you wanted to make excessiveness more explicit in Swahili, you could say something stronger, such as:

• Usiweke mayonesi nyingi kupita kiasi... = Don’t put too much mayonnaise / don’t put an excessive amount of mayonnaise

But the original sentence is natural and idiomatic.

Kwenye is a very common locative word that can mean in, on, at, or to, depending on context.

In this sentence, the natural English meaning is:

• in the salad

So:

• kwenye saladi = in the salad

You may also see other location expressions in Swahili, such as:

• katika saladi = in the salad
• ndani ya saladi = inside the salad

But kwenye is very common in everyday speech.

Because ya links saladi with leo in a way that means of today or today’s.

So:

• saladi ya leo = today’s salad

Literally, it is something like:

• salad of today

This is a very common Swahili pattern:

• chakula cha leo = today’s food/meal
• habari za leo = today’s news

So ya leo is the normal way to express today’s with saladi.

Because in Swahili, adjectives and similar describing words usually come after the noun, not before it.

So:

• mayonesi nyingi = much / a lot of mayonnaise

Compare with other examples:

• mtoto mdogo = small child
• kitabu kizuri = good book
• vitabu vingi = many books

This is one of the basic word-order differences between English and Swahili.

Yes, kwenye is flexible enough that its exact English translation depends on the situation.

For food, English usually prefers:

• in the salad if the mayonnaise is being mixed into it
• on the salad if it is being added as a topping

Swahili kwenye can cover both kinds of location more broadly. In this sentence, the intended meaning is most naturally understood from the context of making salad.