The simple past in English can be translated in two different ways in Spanish (drag the fish into the fish bowl to check the translation:

The Romans spoke Latin.

Los romanos hablaban latín.

Their capital city was Rome.

Su capital era Roma.

In English it is clear from the context that in the first example we are talking about a past habit or custom, while in the second example we are describing a past state or situation. Its equivalent in the present would be the present simple:

Romans today speak Italian.
Their capital city is still Rome.

To translate the following example we would use a different tense in Spanish:

The Romans conquered Hispania.

Los romanos conquistaron Hispania.

In this example we are talking about a finished action; the equivalent tense in the present (that is, for a recently finished action) would be the present perfect:

Romans have elected a new mayor.

Sometimes, however, the context is not enough to make it clear whether we are talking about a finished action (which happened only once) or a past habit (an action that happened repeatedly):

The Romans invaded other countries.

Los romanos invadieron otros países. Los romanos invadían otros países.

To make it clear we must use a special auxiliary verb, used to, which expresses past habits or past states:

Romans used to speak Latin. = Los romanos hablaban latín.
Romans used to invade other countries. = Los romanos invadían otros países.

Notice that we cannot say «Rome used to be their capital city», because Rome still is their capital city.

Similarly, we cannot say «The Romans used to invade Hispania», because they did it only once.

We can also use the modal auxilary would to express a past habit, but not a past state or situation. For past states we can only use used to. If you are in doubt, use used to.

We normally use "would" instead of "used to" when we have already established that the context is about past habits:

The Romans used to invade other countries. First they would conquer them with their army, then they would impose their language and customs. Latin spread this way throughout the south of Europe.

If you bump into a sentence with would where it makes no sense to translate it as a conditional, try translating it as used to:

When I was little I would look at the raindrops as they ran down the window glass, and I'd imagine they were meteorites.

To make the negative and interrogative forms of used to we must always use did, followed by the infinitive form use to:

— Did you use to live in a very rainy part of Spain?
— No, it didn't use to rain as much as it does in Galicia.

Be careful: the pronunciation of use to and used to is the same, so it is easy to make a mistake and say the former when you need the latter (after did).

© Juan José Castaño