Learn Farsi: آن‌ها در یخزار گیر افتادند.

It combines yakh (ice) with the suffix -zār, which indicates a location abundant in something. You might already know this suffix from words like chaman-zār (meadow) or namak-zār (salt flat).

Why is the verb oftādand (they fell) used to mean they got trapped?

It is part of the compound verb gir oftādan (to get stuck or to get trapped). Gir refers to a snag, grip, or catch. When paired with oftādan (to fall), the literal image is falling into a catch or snare.

If gir oftādan means getting trapped, how do I say to trap someone else?

To make the action active, you swap the auxiliary verb oftādan (to fall) with andākhtan (to throw). The compound verb for actively trapping someone or something is gir andākhtan.

Is gir oftādan only used for physical traps like an ice field?

No, it is used metaphorically as well. Just like in English, you can use gir oftādan for frustrating everyday situations, such as being stuck in a traffic jam or trapped in a boring meeting.