Learn Spanish (Latin America): El perro estaba durmiendo mientras estaba lloviendo. - The dog was sleeping while it was raining.

In Spanish, some verbs have a vowel change in their stem when they form the gerund (the -ando or -iendo form). For dormir (to sleep), the o changes to a u, making it durmiendo.

Could I just say El perro dormía instead of El perro estaba durmiendo?

Yes, you can! In Spanish, the imperfect tense (dormía) and the past continuous (estaba durmiendo) can often be used interchangeably to describe ongoing past actions. Using estaba durmiendo just puts a little extra focus on the fact that the action was actively in progress at that moment.

Where is the word for 'it' in estaba lloviendo ('it was raining')?

Spanish doesn't use subject pronouns like 'it' for weather. You simply use the third-person singular form of the verb. So, estaba lloviendo literally translates to 'was raining', but in English, we naturally add 'it' to make the sentence complete.

How does mientras work in this sentence?

Mientras means 'while'. It is used here to link two ongoing actions that were happening at the exact same time. It is very common to see mientras connecting two verbs in the past continuous (like in this sentence) or the regular imperfect tense.