When you hover over the word 'need' in the sentence, the hint gives you 需要, but if you write it in the answer, it's turned down and the correct solution should only have 要. Then either the hint or the 'correct solution' is
Me neither. I tried asking a Chinese friend, and he couldn't really explain it either. But that's like most languages, isn't it? When you speak it as a native, you just "know" what sounds correct without knowing the grammatical rule/idea behind it.
Chinese grammar puts time components first. The sentence could begin with "I" or 我 (the subject), but often the time clause comes before the subject as it makes sense to put the subject next to the action: 我要去。