"The children are crying."
Because its plural.
Ett barn, barnet = a child, the child
Barn, barnen = children, the children
@Zmrzlina, As I understand, there is no such rule for En words and Ett words.
What so you think of the best way to know those Ett words and En words ? Do you have a resources review them, or we just create our own list?
In Swedish you don't need to conjugate a verb for present tense. In English we need The child is crying. "Is" is part of the verb "to be" and I don't think Swedish has that. Barnen gråtar = The children are (that are is not translated) crying OR The children cry