Learn Arabic (Modern Standard): هل الامتحان سهل؟

In Arabic, sentences that describe someone or something (nominal sentences) do not use a word for "is" or "are" in the present tense. You simply put the subject (الامتحان, the exam) right next to the describing word (سهل, easy).

What is the purpose of هل at the beginning of the sentence?

هل is a question word used specifically to ask "yes or no" questions. In English, we often swap word order to ask a question, changing "The exam is easy" to "Is the exam easy?". In Arabic, you keep the normal sentence order and just place هل at the very beginning.

Since this lesson is about ليس, how would I answer "No, the exam is not easy"?

To make a nominal sentence negative, you use ليس (is not). You would answer لا، الامتحان ليس سهلا (No, the exam is not easy). Notice that ليس comes before the describing word to negate it, and it is used in its masculine form to match the subject (الامتحان).

Why doesn't the word for "easy" (سهل) have ال (al-) on the front like the word for "exam" does?

In an Arabic "is" sentence, the subject usually has ال (the) to show exactly what you are talking about. The describing word (the predicate) usually does not take ال. If you added it and said الامتحان السهل, it would just mean "the easy exam" as a phrase, rather than forming the full sentence "the exam is easy."