That's almost exactly the problem. English uses helper words exclusively for future tense, and indeed, helper words like 'to' to form an infinitive. 'Will' is the helper word to show that something is a fact, that it is definite - grammatically, it is indicative. (The sun will rise tomorrow.) 'Would' is the helper word to show that something is an opinion, or dependent on something else - grammatically, it is subjunctive. (If you push that, it would fall; if it was cheaper, I would buy it.)

Spanish has both helper words for future tense (conjugations of 'ir', analogous to 'going to', often used in speech) and straight-up conjugations for future tense (doesn't exist in English; often used in writing). It also conjugates verbs differently if they're indicative, subjunctive, or imperative (asking or telling someone to do something). This is how Spanish manages to have fifty-odd ways to conjugate every verb, which is very confusing to English speakers who make do with three ways and helper words.

Translating a 'future tense sentence' for Duolingo requires you to have psychic powers about whether something is fact or opinion, which helper words are wanted, and so on, and it usually comes down to guessing between multiple 'correct' answers, which Duo will reject all but one of.