You can use a simple formula for telling time:
Ser+ la/las + number/time.
Es la una. → It is one o’clock.
Son las dos. → It is two o’clock.
To ask what time it is you say:
¿Qué hora es? → What time is it?
To ask when something will occur start your question with the preposition a.
¿A qué hora es la fiesta? → At what time is the party?
¿A qué hora vas a la tienda? → At what time are you going to the store?
Here are a few other keywords to know when saying the time.
la mañana → the morning (AM)
la tarde → the afternoon/evening (PM)
la noche → the night (PM)
cuarto → quarter/15 minutes *This is a noun and will always end in o.
media → half *This is an adjective, and since you are referring the the hour (la hora), it will end with a.
la medianoche → midnight
el mediodía → noon
De vs en vs por
Use de when referring to a specific time.
Son las cinco de la mañana → It is 5 o’clock in the morning.
Use en to refer to a general time on a specific day.
Mañana voy a trabajar en la mañana.→ Tomorrow I am going to work in the morning.
Use por to just refer to a general time.
De costumbre, hago la tarea por la tarde. → Usually, I do the homework in the afternoon.
Approaching the next hour
When you have a time that is almost the next hour, there are several ways to communicate the time. It is like in English when you might say it is a quarter to five instead of four forty-five.
Son las cinco menos quince./ Son las cinco menos cuarto. It is four forty-five.
Es la una menos diez. → It is ten to one.
Putting it all together
Es la una de la tarde. → It is one in the afternoon.
Son las dos y cuarto de la mañana. / Son las dos y quince de la mañana. It is a quarter past two./ It is two fifteen AM.
Son las tres y media. → It is three thirty.
*Not that when referring to the one o’clock hour, the verb ser is singular (es). All other hours are plural (son).