Use flash cards... I personally love to use physical things instead of apps
Traer in the preterite tense for third person singular has the -o ending, but traer is an irregular verb in that you must add a j when conjugating this person. Therefore the answer is "trajo."
If you have other problems like this, websites like spanishdict.com are good for checking your work, and creating a table for the verb ("yo traje, t<span>ú </span>trajiste") to figure it out. :)
