"Dad and mom?"
The ו can be pronounced in three different ways, depending on the vowel marks (which are rarely shown on Duolingo, only to clarify alternate spellings). A plain ו makes the "v". One with a dot in the middle (וּ) makes an "oo" sound like "glue". One with a dot on top (וֹ) makes an "oh" sound like "no".
Originally ו was pronounced [w] like the English /w/. Because [w] is the consonantic u, like [j] is the consonantic i, י and ו are used for [i]/[e] and [u]/[o]. In modern Hebrew the [w] sound became a [v] sound. Before, the name of the letter was pronounced wow.
I don't think there's a hard-and-fast rule, unfortunately. Just like asking when do you use c or k in English? You just learn the spellings of words. Just from my observation, though, או is mostly at the starts of words. Few words start with just an וּ. Hope that helps :)