This is the 5584th most frequent Russian word.
поставлены
"Поставлены" means "are placed" or "are set."
In this sentence, 'поставлены' means 'placed' or 'put', describing an action where items (books) were arranged onto a shelf.
Книги были поставлены на полку.
The books were placed on the shelf.
In this case, 'поставлены' refers to tasks being 'set' or 'assigned', implying responsibilities or challenges provided to someone.
Перед ними были поставлены сложные задачи.
Complex tasks were assigned to them.
Here, 'поставлены' involves the placement of artistic elements (accents) within a creative work (the staging).