There are a number of ways of forming the plural in Welsh.
Adding 'au' is quite common.
There are number of other ways.
It's not such an important thing to learn in Welsh because often in situations where you would need the plural in English we use the singular in Welsh.
The main example of this is using numbers with nouns.
eg Apple = Afal
Three apples = tri afal
In this dictionary: http://www.geiriadur.net/index.php?page=ateb&uni=y&prefLang=&term=tegan&direction=we&whichpart=exact&type=noun I found that the plural of tegan is teganau. Here it's tegannau with two "n". Is there an error (in the dictionary or in the course) or are the two forms correct?
The usual plural is 'teganau' with the single 'n', however the form with double 'n' is often found.
Both are accepted as correct answers in the current, in the revised course we'll alter the best translation to be 'teganau' with 'tegannau' as an acceptable alternative.