一般都是固定搭配吧 be able to, able是形容词 be about to about是介词。没注意是否有直接可以加一般的副词以及介词of的用法

Suprised that you never heard of 'it is up to you'? Though up here is an adjective; Yes depends on the way how you think of it, adverbs >>> It IS realLY good, it IS definiteLY you who did it. ie you can not just say 'I am really' if the 'BE' is supposed to describe an object or predicative except something like this >>> Are you OK? Yes, I am indeed. (I am indeed OK. ) Be can be followed by prepositions eg I am like you, I am on it. Sometimes we say it's of high quality, it's of great help. See: http://dict.youdao.com/search?q=be+of&ue=utf8&keyfrom=dict.index

It's up to you!!! 由你来决定 It's up to 是由...决定 ，由....负责 的意思