Llevar o traer / Ir o venir
Although seemingly straightforward, correct use of the verbs ir and venir, and hence llevar and traer, is notoriously tricky. This is due to the subtle difference between the English speaker's and the Spanish speaker's perspective.
In both Spanish and English, these verbs correspond to the notion of 'here' and 'there', i.e. movement to or from another place.
-Venir / traer aquí — Come / bring here
→Ir / llevar allí — Go here/ take there
1) However, in English the speaker may express the notion of movement from the perspective of the listener; in Spanish the perspective of the speaker is always predominant. So, when talking to someone who is in another place, the speaker may use ir in Spanish but 'come' in English.
"No debe emplearse 'venir' con el significado de 'ir', error que cometen algunos hablantes y que hay que atribuir al influjo de otras lenguas, como el catalán o el italiano, en las que el que habla emplea el verbo 'venir' cuando desea expresar la acción de ir hacia su interlocutor". DPD
-[Llaman a la puerta] -¡Voy! — I'm coming [From the perspective of the listener they are 'coming here'; from the perspective of the speaker they are 'going there']
-[Hablando por teléfono] -¿Te vienes a comer? -Lo siento, pero no puedo ir. Estoy ocupado — Are you coming round for lunch? -Sorry, but I can't come. I'm busy
2) When the speaker is referring to a past or future event –when they are no longer/not yet 'here' or 'there'– depending on where the speaker envisages themselves, they may choose ir or venir.
-[En el coche de camino al trabajo] -¿Te vienes a comer a mi casa esta tarde? — Are you coming to mine for lunch this afternoon? [The speaker sees himself at his house (here) in the future, or suggests the listener 'come with' him]
→[En el coche de camino al trabajo] - ¿Vas a ir a comer a mi casa esta tarde? — Are you going to my house for lunch this afternoon? [The speaker sees himself at a different location in the future, e.g. maybe the listener is going to have lunch with the speaker's family]
3) When using a dative pronoun to represent the destination of the movement (me, te, le), first-person singular (me) and plural (nos) tend to call for traer, while second- or third-persons take llevar more easily, since the notion of 'there' & 'them' and 'here' & 'us' is closely linked.
-[Hablando por teléfono] ¿Te vienes a comer? -Sí. ¿Te llevo el cargador para el móvil? — Are you coming round for lunch? -Yes. Shall I bring you the phone charger? / Shall I take the phone charger round (with me)? [In English both are possible]
→Ayer fui a comer a casa de Juan y le llevé el cargador — Yesterday I went to Juan's for lunch and I brought him the phone charger/ I took the phone charger round (with me) [For 'ir +a infinitivo' see discussion]
→Ayer vino Santi a comer a mi casa y me trajo el cargador — Yesterday Santi came to mine for lunch and he brought me the phone charger [First-person is linked to the notion of 'here']
4) When referring to two third-persons, the speaker may choose ir/llevar or venir/traer depending on where they consider 'here' and 'there' to be. Once again, in English the speaker often puts themselves in the shoes of the third-person and uses 'bring', where in Spanish this may sound awkward.
-Cuando regresaba de vacaciones siempre le llevaba/traía regalos — When he returned from his holiday, he would always take/bring presents back for her [In both English and Spanish the speaker may choose 'traer' if they are, or imagine themselves to be, in the destination; 'llevar' would suggest movement away from 'here' to the destination 'there', e.g. another country]
-Ayer Santi fue a comer a casa de Juan y le llevó el cargador — Yesterday Santi went to Juan's for lunch and he brought him the phone charger / took the phone charger round (with him)
-Su madre es quien lleva el dinero a la casa — It's his mother who brings home the money
- See also: ir vs. irse, llevar vs. llevarse
-Llevar o traer / Ir o venir-