I don't really understand.
Do you mean there are grammars and tenses in Japanese spoken like English ( present, past, present perfect tense - see, saw, have seen )
and so Hiragana and katakan are used for the grammar in spoken Japanese.
think 50% of written Japanese are in Hanzi. The road signs are all in Hanzi - you can basically travel around Japan by knowing your written chinese.
Yes, you got it. Hiragana are used for native prounciations without Chinese character equivalents and for grammar, to decline Chinese character root words into tenses and cases. Simple example, 見る = to see (root) 見て = see 見た = saw 見せて = to show. You're also right than more than 50% of Japanese are usually written in Kanji (or Hanzi), as the root words are always preserved as much as possible.
One can always write everything Japanese in Hiragana, but it'd look messy for readability, cause it was not designed to be an independent alphabet system, it's designed as a syllabary to used with Chinese characters. Korean Hangul however, was designed to be an independent alphabet system to replace Chinese characters completely.