"I should visit mother."
Translation:Matrem visitare debeo.
It is because "debere" is a modal verb, and in Latin when you have a modal verb, the main verb must be in its infinitive form.
This is like in English: when you have a modal verb (e.g. can, must, shall) the next verb (e.g. play, eat, drink) must be in its infinitive form. Example: "He can play the guitar." and not "He can plays the guitar." or "He can playing the guitar."
The other two answers are perfectly correct. To add to this, note that a verb in infinitive (‘without [temporal] ending’, the ‘in’ here being similar in function to ‘un’) can function like a noun. Compare this to the difference between ‘to work is good’ and ‘you should work well’.
It can be translated as "should" or "must" in the present. The translation I like to use is ought, because it does a good job of paralleling the verb in translation (i.e. debeo dormire - "I ought to sleep" vs. I should sleep"). Also, debeo is related to the Latin word for debt, and ought is related to the English word "owe", so I like how they both have the image of someone owing something.