"You are going to cover it again."
Translation:Lo vas a cubrir de nuevo.
Although Duo mixes these two tenses, they shouldn't. They might have marked you wrong because in this example, they were looking for the "going to" tense - ir + a + infinitive verb. You are going to cover it.
Cubrirás is the "will" future tense - you will cover it.
Translated into English, these pretty much mean the same thing, but they should NOT be used interchangeably in Spanish. The "going to" tense is much more common, as well as being less "formal" than the future tense.
Yep; although they express slightly different ways of saying things, in practice you can use them all. Here's a quick example: you wouldn't say 'Hi another time!', but rather 'Hi again!'. Also, you wouldn't say 'Until again!', you'd say 'Until another time'. Same with 'for another time', that wouldn't be 'for again'. Una vez más is scarcely used; I think of it as 'A further point in time'. Hope this helped :)
They are exactly the same and both perfectly acceptable.
The second version is easier for English speakers to initially get to grips with but it's my understanding from Spanish speakers (Colombia and Ecuador) that placing "lo" at the beginning is far more popular in spoken Spanish.
Remember that when there is just 1 verb, the object always preceeds it so another reason to get used to this format.