If you were told that en is a Spanish preposition and were asked what it means, you most likely would guess "in" or "on." And you'd be right — in both cases. But en can also be translated as "at," "about," "by," "on top of," "upon," "inside of" and other ways, so its use isn't as straightforward as it may appear.
Fortunately, when en doesn't mean "on" or "in," you can usually tell by the context what is meant. Like some other prepositions, en can seem quite versatile to the foreigner. Here are the most common uses, with examples, of en you're likely to come across:
Indicating location and meaning "in": El dinero está en la caja. The money is in the box. Vivo en España. I live in Spain. (Note: "In" meaning "inside of" also can be expressed by dentro de.)
Indicating time and meaning "in": Llegamos por tren en una hora. We arrive by train in one hour. Trabajaré en el verano. I will work in the summer. (Note: En isn't used with days of the week. Llegamos el lunes. We arrive on Monday.)
Indicating location and meaning "on": El dinero está en la mesa. The money is on the table. Fijó los carteles en la pared. He put the posters on the wall. (Note: The preposition a alternatively could have been used in the second sentence. Fijó los carteles a la pared.)
To express a value: Vendió el carro en $2.000. He sold the car for $2,000. (Note: The preposition por alternatively could have been used in the previous sentence. Vendió el carro por $2.000.)
In numerous expressions, with various meanings: En broma, as a joke; en busca de, in search of; en cambio, on the other hand; en casa, at home; en español, in Spanish; en honor de, in honor of; en la actualidad, presently; en la radio, on (the) radio; en la televisión (TV), on (the) television (TV); en lugar de, instead of; en secreto, in secret; en seguida, immediately; en serio, seriously; en todas partes, everywhere; en vez de, instead of; en vista de, in view of; en vivo, live (as in live TV); en voz alta, in a loud voice; en voz baja, in a soft voice.
With certain verbs to mean "in" or "on": Competer en, to compete in; concentrarse en, to concentrate on; concurrir en, to concur in; confiar en, to trust in; creer en, to believe in; empeñarse en, to persist in; esperar en, to trust in; insistir en, to insist on; intervenir en, to participate in; persistir en, to persist in; reflejar en, to reflect on; resultar en, to result in.
With certain verbs to have other meanings: Adentrarse en, to get into; advertir en, to notice; aplicarse en, to devote oneself to; coincidir en que, to agree that; comprometerse en, to get involved with; concurrir en, to meet at; consentir en, to agree to; consistir en, to be composed of; convenir en, to agree to; convertir(se) en, to change into; equivocarse en, to be mistaken about; extenderse en, to spread over; fijarse en, to notice; ingresar en, to be admitted to; inscribirse en, to register for; juntarse en, to meet at; molestarse en, to bother by; pensar en, to think about; quedar en que, to agree that; reparar en, to notice; vacilar en, to hesitate to.
As a beginner, you don't need to memorize all the phrases and verbs that use en; of the verb phrases, pensar en (to think about) is the most common one that causes beginners difficulty. But you should be aware of those uses, so you know that en might not mean "in" or "on."