In the first story the elves are quite helpful as they help a poor shoemaker finish his tasks faster.
In the second story the elves are quite funny as they make a servant girl spend "three" days together with them in the hollow mountain. Surprisingly when she returns home, she realizes that the elves' three days are actually seven years.
In the third story the elves are quite sneaky as they change a human-child with a changeling.
What is the conclusion you can draw? Be careful with elves because you can never know what they would do.