Two important general facts, for those who have never learned another language before: - The same word can have different meanings in different contexts. For example, in English "bar" can mean an elongated object (e.g. a bar of chocolate or iron) but also a place where you can get a drink, and in legal context it has yet another meaning. - Most words do not have a one-on-one translation into another language. Sometimes this is due to cultural differences, sometimes it is just coincidence. For example, if you know the expression for "bar (of chocolate)", you should not expect that every other meaning of "bar" translates to that same word. In the hints Duolingo gives you the most common "meanings" (in the form of translations) of a given word. Is it clearer now why you cannot just pick the first given translation and apply it every time?
For the future tense, you have to use the present tense followed by the infinitive. Thus "Vi skall spise tidligt" == We shall eat tonight, is more obvious future tense. Vi spiser tidligt could also mean "We eat early" but it seems the point is they want you to recognize that it could in fact mean "We will eat early."
Why is the given correct answer not "We eat early"? "We will eat early" is surely future tense, and we have not yet learned that in Danish.