I need to explain when to use MWEN and when to use MWENS. From what I can see, we use MWEN with an adjective, e.g. li mwen gwo and we use MWENS with a noun, e.g. li gen mwens laj. Is that correct?

You are correct.

But, sometimes you could use MWENS before adjectives.

Li mwens kapab jodi a.

He's less able to day.

M mwens enterese jodi a.

I'm less interested today.

M mwens enkeyte kounye a.

I'm less anxious now.

MWEN is also used when telling time:

It's 2:50. (10 to 3)

Li twazè mwen dis.

It's 6:45. (15 to 7)

Li setè mwen ka.

MWENS (minus, less, fewer) is also used in other situations.

25 - 10 = 15

Vennsenk mwens dis egal kenz.

I need 10 people. No more no less.

M bezwen dis moun. Pa plis pa mwens.

It hurts less today.

Li fè m mal mwens jodi a.

It took them less than 15 minutes to locate the car.

Yo te pran mwens ke kenz minit pou jewnn machin nan.

There are fewer cars on the road on Sundays.

Gen mwens machin sou wout la le dimanch.

Ask me anything