Learn Turkish: Güvenli bir eğlence için çizginin arkasında beklemek zorundasınız. - For a safe amusement, you have to wait behind the line.

Zorunda expresses a strong, external obligation—something you have to or are forced to do due to rules, laws, or physical circumstances. While beklemelisiniz means 'you should/must wait' (a strong recommendation), beklemek zorundasınız means 'you have to wait' because it's a strict safety rule you have no choice but to follow.

Why do we use the full dictionary form beklemek before zorundasınız instead of attaching a suffix to the verb root?

That's the standard rule for forming sentences with zorunda. You always leave the main verb in its full infinitive form (ending in -mAk or -mEk), and then add zorunda followed by the personal copula ending (like -sınız for 'you').

I see the word for 'line' is çizgi. Why is it written as çizginin arkasında here?

To express locations like 'behind', 'under', or 'inside' something, Turkish uses a genitive-possessive structure, literally translating to 'at the line's behind'. Çizgi gets the genitive suffix -nin (of the line). The word for back/behind (arka) takes the possessive -sı (its behind), followed by the buffer -n- and the locative suffix -da (at/on). Together: çizgi-nin arka-sı-n-da.