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

We use هذا because both دفتر (notebook) and كرسي (chair) are grammatically masculine words in Arabic. If we were pointing to a feminine object, we would use هذه instead.

Where is the word for 'is' in this sentence?

There is no word for 'is' or 'are' in the present tense in Arabic. You simply place the two parts next to each other, like هذا دفتر (this a notebook), and the 'is' is understood naturally.

Does هل translate exactly to 'is' here?

Not quite! هل is a special word used to turn any statement into a yes/no question. While it translates as 'is' in 'Is this a notebook?', it functions more like a spoken question mark at the beginning of the sentence.

Why doesn't the Arabic sentence have words for 'a' or 'an' like in 'a chair'?

Arabic does not have indefinite articles like 'a' or 'an'. When a noun like كرسي (chair) is written without a definite article attached to it, it is automatically understood as 'a chair'.