Hola William,

When we have a verb in the imperative with pronouns attached to it, the stress is still in the original stress from the verb conjugated without the pronouns. I mean, for example:

Prepáratelas:

prepara - te - las

The stress on the imperative prepara is on the second "a": pre - pa - ra. So when linked to the pronouns, the stress is still on that "a". The same would apply to compra - te - las.

With compráoslos, this is how it goes:

comprad: imperative (with the stress on the "a", com-prad)

os = pronoun

los = pronoun

In this case, the -d from the imperative for "vosotros" dissappears when having the pronoun "os" attached. But the intonation is still on that "a": com - pra - os - los.

I hope this clarified it.

Saludos

Inma