Learn Arabic (Modern Standard): الامتحان ليس صعبا.

When we use ليس (is not) to negate a nominal sentence, the describing word (the predicate) must change to the accusative case. For a word like صعب (difficult), we show this case by adding the 'an' sound (tanwin fatha) and an extra Alif, making it صعبا.

Why do we use ليس here instead of لا to say 'not'?

In Arabic, لا is mostly used to negate present-tense verbs (like 'I do not go'). To negate a sentence that does not originally have a verb, which is known as a nominal sentence, we must use ليس.

Does ليس change depending on the subject?

Yes! Even though ليس translates to 'is not' or 'am not' in the present tense, it is actually a past-tense verb and conjugates to match the subject. Because الامتحان (the exam) is a masculine noun, we use the default masculine form ليس. If the subject were feminine, we would use ليست.

What would this sentence look like if it were positive?

If you want to say 'The exam is difficult', it would simply be الامتحان صعب. Notice that there is no 'is' verb in Arabic for positive present-tense sentences, and صعب drops the Alif ending because it is no longer being affected by ليس.