"Alors qu'il s'était allongé" or "alors qu'il s'était allongeait"?

Question

I saw the following sentence in the tale "three little pigs" (you have to start somewhere, right :);

Alors que le petit cochon venait juste de finir de construire sa maison et qu'il s'était allongé pour faire une sieste dans son lit en paille […]

As I understand the sentence simply says that once he had finished building his house and has taken a nap – something happened (the big bad wolf arrived, in this case), and so it makes sense that it uses the imperfect tense. If so, why does it use "allongé" and not the imperfect form "allongeait"?

These tenses still confuse me.

Answer 1

It's something called Passé composé and Plus-que-parfait.

It happens in English too: the present perfect is "I have lain down". The past perfect (quite similar to the pluperfect) is "I had lain down". "Lain" is the past participle which doesn't inflect.

Similarly for continous forms, "lying" is the present participle which doesn't inflect, for example:

• I am lying
• You are lying
• He is lying

In this French sentence the verb isn't "allongeait", it's "s'était allongé" (had lain down/on the bed), "allongé" is the past participle used in the plus-que-parfait.