"The elephants are big."
Translation:Les éléphants sont gros.
Because that translates to tall rather than big. Grands/grandes big in height (tall), gros/grosses = big in width (fat) and big in general. While your sentence is factually true as elephants are big in height, the Duo wants us to describe the size of elephants more generally.
Edit: Actually I am mistaken, having just re-tried successfully with "les elephants sont grands" it seems that they have an objection to the feminine form.
Why not "Des elephants sont gros?" If you say in English, The elephants are big, it seems you refer to a certain group of elephants, therefore an indefinite number of them. Would love to have some help with this. I just completed reading, note-taking and a test on about.com/French on this subject, but this item has me perplexed.
The elephants, in English, refers to a definite group of elephants, a group that is known in some way to the participants in the conversation. Those elephants right there, the ones that we were talking about, the ones that have been marauding in our fields. We don't know everything about them, how much each one weighs, whether they are hungry, exactly how many there are in the group at any point in time. All kinds of things that could be known about the group are not available to us but we still know generally which group we are talking about. In this sentence someone is identifying a feature of the group of elephants that we know about that may have escaped our attention.
Some elephants is indefinite. We are being told that some but not all elephants are big. Some elephants may include elephants that we know about or it may not. Some elephants may include elephants that we don't know about or it may not. It is indefinite. We don't know which animals are included and which are excluded, only that they are elephants. If we do want to know, we have to ask for more definite information.
The example here gives the English as the elephants. That means the speaker believes the lister/reader knows which group of elephants he is referring to. Some elephants means the speaker believes the reader/listener does not know which elephants are being referred to.