要 yao4=want, will, must

 要 is always badly taught I have never seen one single good introductory lecture on the various uses of 要！ 要  so often means "will" or "must" that "want"... I would Only learn it as 想要 = would like, want.
Just on it's own you really don't know what that yao means. 
需要 = need require.
Just yao alone can mean “intend to”, “to be obligated to” (whether by society or natural forces), or “want”, but to avoid ambiguity use xiangyao （would like) or xuyao (need), which is also somewhat more polite.
Though you can form a future tense in Chinese with yao, you are better to  just add a time modifier 明天我要去美国 to disambiguate it (and then the yao is likely redundant). I think it's better to use hui 会 for the future "will" than yao since even with a time colocation yao could mean "must" rather than "will".

daodi literally means "to get to the bottom of" (something); this co-location in English is really the same exact thing.  到底 though of course a better translation depending on context might be "in the final analysis"