这两句，【要】都是表示情态的动词。表示了"必须完成某事，应该完成某事，希望完成某事'的意思。情态和英语need类似 1。吃饭以前,要洗手-Before you eat, you need to wash your hands. 2。还要走开车多久才能到？-How much time do you need？for you to get there by car.

要 has a lot of meanings: need : 我 要/需要 用一下你的汽车。 I need to use ur car. want or hope: 妈妈我 要 去动物园。 mom i want to go to the zoo. or mom i hope to go to the zoo. should: 吃饭前 要 洗手。 wash ur hands before you eat. ask: ask somebody to do something. 他 要 我借给他10美元。 he asks me to lend him 10 dollars. in ur example, 要 in 还要 means need 还要 多久才能才能吃饭？ it means he has been waiting for a while then he wants to know how long he needs to wait yet. then we must use 还要 还 means yet if we say 要多久才能吃饭？ it means he starts waiting and he wants to know how long he need to wait.

要 means：have to ，must，need ； 还要 means：still must ，still need，still have to

吃饭以前,要洗手-Before you eat, you should wash your hands. 要=should 还要走开车多久才能到？ 要 is word group 还要 = sitll

"要- i have learnt that this means 'to want' ," here goes the basic mistake because "to want" = 想 while "要" = future intention, as "will" in English