Learn Turkish: Lütfen bu ciddi sorunu hafifsemeyin. - Please do not take this serious problem lightly.

The root is the adjective hafif (light). Adding the attitude suffix creates the verb hafifsemek (to perceive as light / to underestimate). From there, we add the negative suffix -me, a buffer -y-, and the formal/plural imperative (command) suffix -in, giving us "do not take (it) lightly."

The lesson is about the -(I)msEmEk suffix. Why is there no "m" in hafifsemek?

This suffix actually has two common variants: -(i)mse (as in küçümsemek from küçük, or benimsemek from ben) and the shorter -se (as in hafifsemek or garipsemek). Both do exactly the same thing—they turn a noun or adjective into a verb meaning "to perceive as [root]." The variant used is fixed for each word, so you just need to memorize the resulting verbs.

Why does sorun (problem) take the -u suffix to become sorunu here?

The verb hafifsemek takes a direct object because you have to take something lightly. Because the object here is pointed out with bu (this), it is a specific, known object (bu ciddi sorun). Therefore, it requires the definite accusative case suffix, which is -u here due to vowel harmony.

Does hafifsemek always mean "taking something lightly" in a negative, dismissive way?