"Τον" can be: The masculine definite article in the accusative case, just like in the exercise -or- the masculine object pronoun (as in, "I help him").
"Το" can be: The neuter definitive article in both the nominative and accusative case -or- the neuter object pronoun (as in, "Forget it").
The accusative case usually answers the question what?/who?/which? (It might be a good way to remember when to use it.)
What did you touch? The elephant. - Τι άγγιξες; Τον ελέφαντα.
Who did you see? Mary. - Ποιον είδες; Την Μαρία.
Which of the two did you choose? The red one. - Ποιο από τα δύο διάλεξες; Το κόκκινο.