French verbs are, for historical reasons, typically grouped into three classes. The loss of final consonants in French has resulted in a serious divergence, wherein the verb conjugation system of the spoken language is significantly simpler than that of the written language.
French (informal 2nd person) imperatives for almost all verbs, and in particular all class I and II verbs, are pronounced identically to the second person present.
However, presumably representing an earlier sound change, the spelling rules for French verbs in Class I are different; the present form ends in a silent s.
Tu retournes chez toi. /ty rəturn ʃe twa/ "You are returning home."
Retourne chez toi. /rəturn ʃe twa/ "Return to your home!"
However, before the pronouns y and en in the imperative, an "s" is pronounced (as a /z/, due to other historic sound changes that I think aren't relevant to this question). This "s" is also obligatory in the orthography:
Retournes-y /rəturnzi/ "Return there!"
For class II and III verbs (ignoring some highly irregular ones for simplicity), the s is represented in the orthography in all situations; it is usually silent but it is pronounced (again as /z/) before y and en.
A reasonable hypothesis would be that the orthographic [s] was historically pronounced in the case of Class II and III verbs, and not in the case of Class I verbs, and then was lost in word final position but retained in the "liaison position" before "y" and "en." (This is typical of French sound changes.)
However, this contradicts the evidence that the [s] is also pronounced in the liaison position for Class I verbs. This could be a recent development by analogy, but it would be a little surprising, since the Class I verbs are more common and more regular. (In addition, it would be surprising for a relatively recent development to have made its way into the standard orthography as it has.) Does anyone know where this came from?