Learn Swedish: Förstår du problemen? - Do you understand the problems?

This is a very common trap! For en-words, adding -en to the end makes them singular and definite (like bilen for "the car"). However, problem is an ett-word. For ett-words that end in a consonant, adding -en makes them definite plural ("the problems"). If you wanted to say "the problem" (singular), it would be problemet.

If problemen means "the problems", how do you say just "problems"?

The indefinite plural is just problem. In Swedish, ett-words that end in a consonant do not change when they become plural. So you say ett problem (a problem) and flera problem (several problems).

Why is there no word for "Do" in this question?

Swedish doesn't use a "helper" word like "do" to form questions. Instead, you just swap the order of the subject and the verb. The statement Du förstår (You understand) simply becomes the question Förstår du? (Understand you? / Do you understand?).

Why does förstår end in an -r?