Learn Farsi: بچه عقبِ اتاق است. - The child is at the back of the room.

Structurally, yes! Location words like عقب (back/behind) use the ezafe vowel (the "e" sound) to link to the noun that follows, just like سمتِ (samt-e) does. However, notice there is no به (be / to) here. That is because the child is already located at the back of the room, not moving towards it.

I see a small diagonal line under the word عقبِ. What is that?

That symbol is a kasra, and here it represents the ezafe (pronounced "e"). It links عقب (back) to اتاق (room) to mean "the back of the room." Normally, this short vowel is not written out in everyday Farsi, but texts often include it to help learners see exactly how the words connect.

Why is there no word for "at" or "in" (like در) before عقب?

In Farsi, you do not strictly need a preposition like در (dar) before spatial nouns like عقب (back/behind), جلو (front), or بالا (above). While you could technically say در عقبِ اتاق (dar aghab-e otaagh), simply dropping the "dar" and saying عقبِ اتاق is much more natural and common in everyday speech.

How do we know the translation is "The child" instead of "A child"?

Farsi does not have a direct equivalent for the definite article "the." When a noun like بچه (bache) is used without an indefinite marker—such as the suffix (i) to make (bache-i / a child) or the word (yek / one)—you can assume from the context that it refers to a specific, known child.