"I have to work on Sundays."
Translation:Tengo que trabajar los domingos.
It's the first person of the verb 'deber'. Their answers sometimes are not in context of what you are learning. Another answer would be "Tengo que trabajar yo los domingos."
'Tener' is to have (e.g. 'Tengo dolor de cabeza' => 'I have a headache.') and 'tener que' is to have to (e.g. 'Tengo que tomar una pastilla.' => 'I have to take a pill.'