Unlike in English, Spanish has two distinct tenses which are used to talk about past events. One is called the preterite and one is called the imperfect. The confusion arises when people try to use them and find out they can NOT be interchanged and used however one pleases. Changing the tense will change the meaning of a sentence and could potentially cause real confusion. If we take the first sentence on this page we can begin to see some of the different uses. "I used to like grammar" = imperfect / "I learned about the past tense." = preterite. So even in one sentence we can have combinations of these two tenses.
I can hear the screams of frustration all the time. I get it, this is a really tricky topic and it can be really frustrating when it feels like you're just trying to guess which situation needs one and which needs another. Lets take a deep breath, maybe another, just one more for good measure. Ok, now we're ready to talk about Preterite vs. Imperfect.
If we imagine that each shape on the following timeline is a different event we can see several different things.
One thing we notice is that there was only one green triangle. This singular event occurring in the past is a specific instance. If we were talking about the green triangle we know exactly which time we're talking about because there's only one of them. So, what would be used to describe this event? The preterite, this is a specific event that began and ended in the past.
Another thing we might notice is the series of purple squares that happened later on. If we wanted to talk about the series as a whole, something that regularly occurred, which one would we use? The imperfect, any time you would use the phrase "used to" you would use the imperfect. If this event were swimming in the lake, I would very likely say, "I used to go swimming in the lake." If I said "I went swimming in the lake" this would imply only going one time, which is a singular event, which would use the preterite.
Finally we have two events that happened at the same time, better yet, the purple diamond interrupted the orange cloud. So with two events what would we use to describe these events? This is a little bit tougher, for the single event of the diamond we use the preterite. This is because we are talking about the beginning of an event, which fits into the preterite. For the orange cloud going on behind it though we would use the imperfect. We're not talking about the beginning or the end of an event, and it's not clear if this was a single event or not so the preterite doesn't fit this event.
Single events
Actions in a chain of events
Actions that reference the beginning or the end
Repeated as part of a habit
Time and dates
Referring to age
Descriptions and Characteristics
Emotional or Mental states
Fui a la biblioteca ayer.
(I went to the library yesterday.)
Montaron la bicicleta, jugaron basquetbol, y trabajaron en el jardin.
(They rode bikes, played basketball, and worked in the garden.)
¿Hablaste con tú papá a las dos?
(Did you talk with your dad at two?)
Cuando era niño, jugaba con legos.
(When I was a child, I used to play with legos.)
Él leía un libro cuando yo entré.
(He was reading a book when I entered.)
Cuando ella tenía dos años, era muy hermosa.
(When she was two years old, she was very pretty.)
Casarse = to get married
Cumplir años = to turn a certain age
darse cuenta de = to realize
decidir = to decide
Descubrir = to discover
Graduarse = to graduate
Llegar = to arrive
Morir = to die
Nacer = to be born
Salir = to leave
Una vez - one time
ayer - yesterday
anteayer - the day before yesterday
ayer por la mañana - yesterday morning
anoche - last night
esta mañana - this morning
la semana pasada - last week
el año pasada - last year
a menudo - often
rara vez - rarely
generalmente - usually
algunas veces - at times
tantas veces - so many times
cada día - every day
todas las semanas - every week
Just in case this doesn't make a whole lot of sense for your here are some other articles that might be clearer.
This is a really good one, I actually used a lot of the information there to create this write up.