These are two words that can be really confusing for the English speaker because they are false friends. First let’s review what the meanings are in English of assist and attend.
Assist is to help. This could be used to help another person or it could be a machine that lends support.
Attend is to be present at an event. Outside of the normal thinking of attending a show, it could also be that a fever could attend a cold. A fever is present with the cold. Though not its primary meaning in English, it is used to mean to take care of or give help to as in the example of “the nurse attended the patient.”
In Spanish these words carry the other’s meaning.
Asistir is to be present at, or to regularly attend.
Atender is to help out as in a nurse helping a doctor in surgery.
This is a pair of words that may take a person quite some time to realize that they are using them wrongly. It would be easy to assume that they carry the exact same meaning as they do in English. While both words in English can mean to be present and to help, they each have their primary usage which is opposite their primary usage in Spanish.