"Listen to me!"
Translation:Gwranda arna i!
No, nothing to do with the several dialects.
ar is used with a number of verbs to modify or enhance their meaning. Here, gwrando ar.. means 'listening to...'. You also edrych ar (looking at), for example.
Is "Gwrandwch arna i!" an acceptable answer? I can't figure out if I spelled something wrong or if the grammar is incorrect, but my answer wasn't accepted.
Gwrando is an irregular verb. The stem is gwrandaw-. The chi-form of the command is Gwrandewch!, where the -a- at the end of the stem has changed to an -e-.