أين هو هذا الدرس في الكتاب؟

Yes. أين (where) is an interrogative word, and it typically comes at the beginning of the question in Modern Standard Arabic (MSA).
So أين ...؟ = Where ...? is the standard way to form a where-question.

هو is the pronoun he/it and here it functions like a copula (roughly like is) in a nominal sentence. In MSA, you can say:

• أين هذا الدرس في الكتاب؟ (often acceptable, especially in less formal style)
• أين هو هذا الدرس في الكتاب؟ (very common and clear)

Including هو can make the sentence feel a bit more explicit: Where is this lesson in the book?

Because الدرس (lesson) is masculine in Arabic. The pronoun used to refer to it is therefore هو.
If the noun were feminine (e.g., الصفحة the page), you might use هي:

• أين هي هذه الصفحة في الكتاب؟

هذا means this (masculine singular). In Arabic, demonstratives typically come before the noun:

• هذا الدرس = this lesson

For feminine nouns you use هذه:

• هذه الوحدة = this unit (since وحدة is feminine)

In Arabic, when you use a demonstrative like هذا/هذه, the noun is normally definite, and it commonly appears with الـ:

• هذا الدرس (standard)

So Arabic often uses both: demonstrative + definite article. It’s the normal pattern in MSA.

في is a preposition meaning in.
الكتاب is the noun the book, and after a preposition it becomes the object of the preposition (in traditional grammar: majrūr, “genitive”). So:

• في الكتاب = in the book

It’s a prepositional phrase telling you where the lesson is located.

In fully vowelled, very formal MSA recitation you may see/pronounce:

• أينَ هو هذا الدرسُ في الكتابِ؟

But in most modern speaking and much everyday reading, learners often don’t pronounce final case vowels, especially in unscripted speech:

• أين هو هذا الدرس في الكتاب؟ (common pronunciation)

Both are useful: learn the formal endings, but don’t be surprised that they’re often dropped in practice.

It’s a correct, natural MSA question. Alternatives you might also see:

• أين يقع هذا الدرس في الكتاب؟ = Where is this lesson located in the book? (more formal)
• في أي صفحة هذا الدرس؟ = On which page is this lesson? (more specific)
• أين أجد هذا الدرس في الكتاب؟ = Where can I find this lesson in the book?

Not always. You can structure the question in different ways. Common patterns include:

• أين + المبتدأ؟ (Where + subject?)
  أين الدرس؟ = Where is the lesson?
• أين + هو/هي + ...؟
  أين هو الدرس؟ = Where is the lesson?
• أين + يوجد + ...؟
  أين يوجد الدرس؟ = Where does the lesson exist / Where is the lesson?

Using هو/هي is common, but not mandatory in every phrasing.

Arabic nouns have grammatical gender. Some clues:

• Many feminine nouns end with ـة (taa marbuuTa), e.g. مدرسة (feminine).
• But many nouns don’t show gender clearly, so you learn it with the word.

For this sentence:

• درس is masculine → هذا, هو
• If it were feminine, you’d use هذه, هي

A good habit is to learn nouns with a matching demonstrative: هذا درس, هذه صفحة, etc.

Yes, if you mean in a book (not a specific known one), you can make it indefinite:

• أين هو هذا الدرس في كتاب؟ = Where is this lesson in a book? (less common in real conversation)

Usually in context you mean a specific book, so في الكتاب (in the book) is the natural choice.