Learn Arabic (Modern Standard): هل هذا كرسي؟ لا، هذه طاولة.

In Arabic, nouns have a gender (masculine or feminine), and the word for "this" must match the noun's gender. كرسي (chair) is masculine, so we use هذا. طاولة (table) is feminine, so we use هذه.

How can I tell that طاولة (table) is feminine?

You can tell by looking at the last letter. It ends in a Ta' Marbuta (ة), which looks like a tied-up circle with two dots. This is the most common sign that a word is feminine in Arabic.

Where are the words for "is" and "a" in "Is this a chair?"?

Arabic doesn't use a word for "is" (the verb "to be") in the present tense, and it doesn't have a word for "a" or "an". So, هذا كرسي literally translates to "this [is a] chair".

Does هل mean "is"?

Not exactly. هل is a question word used at the beginning of a sentence to turn a statement into a yes-or-no question. Since Arabic doesn't use "is" here, هل simply does the job of signaling that you are asking a question.