For one o'clock use "Es la una"; for every other hour use "Son las" plus the number, because the verb agrees with the hour. Add minutes with "y", or subtract them from the next hour with "menos".
Examples
- Es la una y media.
- It's half past one.
- Son las tres y cuarto.
- It's quarter past three.
- Son las ocho menos diez.
- It's ten to eight.
The mistake to avoid
Learners say "Es las tres" - only one o'clock takes the singular es.
Where this fits in
Numbers, days, time, Spanish A1: Count, talk about days and months, give the time, and handle numbers in real situations.