Breakdown of Fueron esos mentirosos los que se burlaron del niño.
ser
to be
el niño
the boy / the child
ese
that (m.sg., also esa, esos, esas, eso)
el que
which (after preposition) / the one who (in emphatic sentences)
Questions & Answers about Fueron esos mentirosos los que se burlaron del niño.
Why does the sentence start with the plural Fueron instead of the singular Fue like the English "It was"?
Could we use quienes instead of los que in this sentence?
Yes! When referring to people in a cleft sentence, you can use either el que / la que / los que / las que or quien / quienes. Saying Fueron esos mentirosos quienes... would be perfectly correct and sounds a bit more formal.
Why is there a se in se burlaron?
Why does it say del niño instead of just al niño?
The verb burlarse is always followed by the preposition de to indicate who or what is being mocked, much like "to make fun of" in English. When de is followed by the masculine article el for el niño, they must combine into the contraction del.