Most texts and teachers of Spanish are quick to point out the false cognate asistir -- that it does not mean to assist, as in to lend someone a hand. It always is used with the preposition a and means to attend, as in to attend class.
Juan siempre asiste a sus clases de química (John always goes to his chemistry classes).
This fact logically leaves some students wondering how to express the idea of helping someone out. The answer vexes many of them, but it needs to be learned as a dull fact of vocabulary: one solution is atender. It is as if the two cognates reversed their roles between the two languages and that is as good a way to remember their difference as any other. Rather like the fact that librería is a bookstore but biblioteca is library just as how sopa is soup but soap is jabón (not to be confused with jamón which is the meat portion of a ham & cheese sandwich!). The a used with atender is not used in the same way as the a with asistir -- this use is the personal a:
Los dependientes atendieron a los clientes en el almacén (The clerks helped/tended to the customers in the department store).
The verb atender can also be used in the sense of tending to or minding one's affairs . Note that the a is still required, although it is not the personal a, but simply a prepositional marker:
Ellos siempre atienden a sus asuntos sin mirar a nadie (They always take care of their own business without looking at anyone).
Finally, there's the verb to help: ayudar. It is the generic way of expressing most of the English meanings of the verb to help, but not all.
Ellos me ayudaron cuando no tenía trabajo (They helped me when I didn't have a job).