Rules for the prefix 'U' in front of names

[Carol m]

In some sentences & questions Duolingo uses [for example] UBen or USarah when using someone's name but at other times, names are just Ben or Sarah.
Is there a rule to tell when to use the 'U' prefix in front of names and when the correct answer is simply the name without the prefix?

[SarahT]

Yes! There is a rule! You remove the "u" when you are addressing someone directly. Grammatically this is the vocative case (in English, the vocative case is identical to all the other cases, so we don't notice). You remove the prefix when you are speaking to someone, rather than speaking about them.

So:
Sawubona, Sarah! - Hello, Sarah!
USarah uhlala - Sarah sits.

The same pattern applies to words like ugogo (grandma) and ubaba (dad). It also applies to nouns in other classes, too, like: Sawubona, nyoni! Hello, bird!