I'll do "what I can (do)" to help you.
引號中的子句是當作動詞do的受詞的名詞子句。因為要避免動詞重複,而將子句的動詞省略,只留下助動詞can。
what是「帶先行詞的關係代名詞」,兼名詞子句的「受詞」。what = the thing that
to help you是「不定詞片語」,當作述部will do xxx的「修飾語」。不定詞是verbal(動狀詞)的一種,是由動詞(verb)衍生的詞類,但用法不同於動詞。
what I can-----------adv.clause +root verb="can" preceded by will do(=VO).
I'll do------------------omit object----SVO
-------do=aux.verb used before a verb (kill) for emphasis
------=do away with=phrasal verb=(kill)
to help you----------infinitive phrase as adverb phrase qualifying "do",no need root verb="can"