"Mae hi'n dawel heno."

Translation: It is quiet tonight.

[Doshia2]

Would "it is quiet" normally be masculine or feminine?

[DaibhidhR]

If it is clear what noun is being referred to then you use the appropriate gender. For example, if someone said

Roedd y môr yn stormus y bore yma
The sea was stormy this morning

and you wanted to reply

It is quiet tonight

you would use the correct pronoun for môr (which is e/o), so

Mae e/o'n dawel heno

But if you are making a general statement about the state of the world, the weather, etc. without referring to a particular noun, the notes say, in the 'Weather' section

for an abstract 'it', such as when referring time, distance or weather, we use the pronoun hi

This is an arbitrary convention. The cousin languages of Gaelic (e) and Irish (sé) use the masculine.

[quiltmage]

how would you know it is "it" rather than she in this sentence?

[DaibhidhR]

You don't. They should accept both answers. There are no reports here of she being rejected, so normally I would assume they do accept her. However, there are two posts missing on this page. That is, the number of posts visible is two less than the number claimed. So who knows what was in the other posts.

[ibisc]

In the wild, just by context.

As there is no context here, though, both 'she' and 'it' are accepted in translations.