Learn Spanish (Latin America): Cuando llovía, los caballos no trabajaban en el campo. - When it rained, the horses did not work in the field.

The imperfect tense is used to describe past habits or repeated actions. Because this sentence is about what the horses used to do every time it rained, rather than a single specific event, both verbs take the imperfect.

What is the infinitive of llovía, and is it irregular?

The infinitive is llover (to rain). In the present tense, it has a stem change to llueve. However, in the imperfect tense, it is completely regular! You just drop the -er and add the regular -ía ending to get llovía.

The English translation says 'When it rained', which looks like a completed action. Why not translate it as 'Whenever it rained'?

English often uses the simple past ('it rained', 'they did not work') for both one-time events and past habits. Spanish strictly separates them. You can definitely think of it as 'Whenever it rained' or 'When it used to rain' to help you remember why the imperfect tense is used here.

Can el campo mean 'the countryside' as well as 'the field'?

Yes! The word campo can mean a specific agricultural field, but it is also the general word for the countryside. Depending on the broader context, the horses might be avoiding a specific field, or they might just not be working out in the country.