Learn Arabic (Modern Standard): السبب ليس واضحا.

ليس is used specifically to negate nominal sentences—sentences that describe what something is, without a main action verb. Because "The reason is clear" is a nominal sentence, we must use ليس to say "is not". We generally use لا to negate present tense action verbs.

Why is there an extra alif at the end of واضحا?

When you use ليس, the word describing the subject (the predicate) changes its case to the "accusative" case. For indefinite masculine words, this is shown by adding the "-an" sound (fathatayn) and a silent alif at the end. So, واضح becomes واضحا.

Would ليس stay the same if the subject were a feminine word?

No, ليس conjugates (changes form) to match the subject, just like a verb. Because السبب (the reason) is masculine, we use ليس. If the subject were feminine, like الصورة (the picture), it would change to ليست (laysat).

I remember something about sun and moon letters. Does that affect how we pronounce السبب?

Exactly! The letter Seen (س) in السبب is a "sun letter." This means the "L" sound in the definite article الـ (al-) gets skipped, and you double the "s" sound instead. You pronounce it "as-sabab" rather than "al-sabab".