The "a" in many verb phrases conveys no specific meaning. For example, it's used with a form the verb "ir" to express an event in the future, like "Yo voy a estudiar." The "a" is also used before a direct object that is a person: "Yo voy a llamar a mi amiga." The "a" can also assume its common definition, "to," in sentences like "Yo voy a la escuela" or "Yo voy a San Antonio." Some expressions involving time also depend upon "a": "La fiesta es a las nueve."
The "en" in Spanish sentences typically means "by," "on" or "in." It can convey location or place: "Yo estoy en el parque" or "El libro está en la mesa." Generalizations about subjects also use "en": "Mi amigo es bueno en matemáticas." With the verb "ir," "en" relays method of travel: "Yo voy en coche." In time expressions, "en" means "in": "Yo voy en el verano."