Learn Arabic (Modern Standard): هذا ليس خطأ.

In Arabic, لا is mostly used to negate action verbs. However, sentences describing what something "is" or "are" do not have a main action verb; they are called nominal sentences. To negate a nominal sentence and say "is not", we must use ليس.

If ليس means "is not", what is the Arabic word for "is" in the positive version of this sentence?

There is no word for "is" in the present tense in Arabic! To say "This is a mistake", you simply put the two words together: هذا خطأ (literally: "This a mistake"). You only need a linking word when you are making the sentence negative with ليس.

Does ليس change its form depending on what we are talking about?

Yes, it behaves like a verb and must match the gender of the subject. Because هذا (this) is a masculine word, we use the masculine form ليس. If we were talking about a feminine noun, we would use the feminine form ليست.

Can I change the word order and start the sentence with ليس?

Yes, both word orders are perfectly fine. You can say هذا ليس خطأ or start with the negation: ليس هذا خطأ. Arabic is flexible, though starting with هذا slightly emphasizes the word "This".