The verb ser agrees with the hour itself: use é only with uma hora, meio-dia and meia-noite, and são with every other hour. Add minutes with e, and take them away with para as.
Examples
- É uma hora.
- It's one o'clock.
- São três e meia.
- It's half past three.
- São dez para as nove.
- It's ten to nine.
The mistake to avoid
Saying "É duas horas" - any hour above one is plural, so it must be são duas horas.
Where this fits in
Numbers, days, time, Portuguese A1: Count, talk about days and months, give the time, and handle numbers in real situations.