"The boy sang a pretty song."
Translation:El niño cantó una canción bonita.
"Canté" -> "I sang"
"Cantó" -> "he/she/you sang" (Note the accent! It makes a big difference between first person singular present tense and third person and second person formal singular past tense)
In this case, "el niño" is third person singular, hence "cantó".
(sorry for cluttering your email! There's no way to edit it on the app. I had to delete this two times to edit it)