Hola R,

when pronouns are added to a verb in the infinitive or the imperative, the stress stays in the original syllable, so, using these examples we have:

com-prar (infinitive always stressed on the last syllable)

then, after adding the pronouns, the word is still stressed on that -a (-prar)

com-prár-se-los

Because the word is now longer adding two extra syllables, and the stress in on the third to last syllable, following the rules of accentuation in Spanish you need the written accent. 

The same goes for cómpreselos:

com-pre (imperative with the stress on the first syllable com)

We add the pronouns and keep the intonation on the same syllable:

cóm -pre-se-los 

Again, following the rules you need the written accent. 

We mention this in this lesson about the use of pronouns with the imperative. You can have a look and see some examples with imperatives adding pronouns. There is also a link by the end that says "written accents" that will take you to a general explanation about accents in Spanish. 

Saludos

Inma

Hola R,

it happens to most verbs in the imperative with pronouns attached, but that will depend on the word each time; once the pronouns are attached and becomes a longer word with more syllables, the presence or absence of the written accent will depend on the rules for accentuation in Spanish. For example, the imperative of ir + the pronoun "os" is "idos" (id + os). Here the stressed vowel is -i, but that -i is on the penultimate syllable, and that word ends in -s, so there is no written accent. Here is a lesson on accents that will help.

As per notifications, I will pass that on so they can check. 

Saludos

Inma