"The soldiers and tourists hurry off the high grass."
Translation:A katonák és a turisták lesietnek a magas fűről.
It follows the direction given with the suffix -ről. The soldiers go "down from" the grass, so you can complement that with using the le- prefix. I think using el- here might sound a bit odd. (Not a native!) El- is mostly used when going "away from" something (so typically with -tól/től) or when given a target in the meaning of "go over to".
Correct. "El-" is also used when you are not in or on something but, for example, next to something. You cannot some OFF being next to something, rather, you come AWAY from that position.
Btw, all this usage of "lesietnek", and "sietnek" in general, is highly unusual, weird. The verb "siet" is used much less than it is suggested in this course.
Doesn't have to be. Remember that the focus of the sentence is on whatever is in front of the verb stem. So in the sentence "A katonák sietnek le", the focus is on the person; it answers the question "Who is hurrying off?" On the other hand in "A katonák lesietnek", the focus is on the verb/action itself, so it answers the question "What are the soldiers doing?"