asistir vs atender
'Assist' is a trap. Asistir means to attend an event; atender means to serve someone or pay attention.
asistir
To attend (an event, class, meeting) — asistir a. Also to give care to someone, formally.
Asistí a la reunión.
I attended the meeting.
Always asistir a. This is the everyday meaning. 'Attend' is the English match, not 'assist'.
Asistieron más de cien personas.
More than a hundred people attended.
Used for turnout at events.
Los médicos asistieron al herido.
The doctors treated the injured man.
Formal medical sense of providing aid. Also 'asistente' = assistant.
atender
To serve, to see to someone, to answer a call, or to pay attention.
¿Ya le atienden?
Are you being served?
The shop assistant's question. Atender = to serve a customer.
Atiende, por favor.
Pay attention, please.
In many regions atender also means pay attention. In Spain, 'prestar atención' is more usual.
No pude atender el teléfono.
I couldn't answer the phone.
Atender una llamada = to take a call.
Tricky bit
To say 'help' use ayudar. Learners who say 'asistir a mi madre' mean to attend her like an event — unless they mean caring for her in a formal medical sense.