This is a phrase from the opening lines of the Metamorphoses. (1.1–4) I am curious about a couple of things when it comes to this phrase. First, mutastis is an alternative form of the second-person perfect plural. How did this alternative come about? Also, which grammar references should I look at to find these alternatives?
The literal translation of the phrase is "For you have changed them also". But what exactly does Ovid mean by this? I have reproduced the context, below, along with my English translation.
In nova fert animus mutatas dicere formas
corpora; di, coeptis (nam vos mutastis et illas)
adspirate meis primaque ab origine mundi
ad mea perpetuum deducite tempora carmen.
My mind inclines me to speak of bodies changed into new forms; gods, breathe on my undertakings (for you have changed them also) and from the first origin of the world to my age lead my perpetual song.
I appreciate any help in understanding what Ovid means by this phrase and the peculiarity of its verb mutastis. Thanks!