EN or A? 1

The movement vs location distinction — The key to choosing between a and en is movement: a = going toward a destination (motion verb involved); en = being at a location (static). Voy a la tienda (movement) vs Estoy en la tienda (location). Check the verb: does it imply movement?

En for non-vehicle transport — Use en for enclosed transport (you're in the vehicle): en coche, en tren, en avión, en barco, en taxi. For open or self-powered transport use a or nothing: a pie, a caballo, en bici. This contrasts with English "by" for all transport.

Al + infinitive — upon doing — Al + infinitive (a + el contracted before an infinitive) means "upon doing" or "when doing": Al llegar a casa, llamó (Upon arriving home, he called). This construction is very common in Spanish and has no direct English structural equivalent — it's worth learning as a fixed pattern.