A clock time takes a les: a les deu. A span uses de... a. Before and after doing something are abans de and després de, and both are followed by an infinitive rather than a conjugated verb.
Examples
- Tinc hora a les deu i mitja.
- I have an appointment at half past ten.
- Treballo de nou a sis.
- I work from nine to six.
- Truca'm abans de sortir.
- Call me before you leave.
The mistake to avoid
After abans de the verb stays in the infinitive: abans de sortir, not "abans de surto".
Where this fits in
Plans i cites, Catalan A2: Proposar alguna cosa, concertar i ajornar una cita, i parlar del temps que canvia els plans.