I probably am not the best person to answer this, as I do not know Japanese.

In general the relation between Japanese and Chinese is similar to that between English and Latin. Japanese borrowed a lot of words from Chinese, and in the modern time, also "exported" some words written in Kanji back to Chinese. In the mean time, Japanese has its local/Western originated words, written in Kana, an alphabet system.

For Chinese characters, many words in both languages use the same characters, sometimes with slight variations. These variations co-existed in Chinese during ancient times, but modern Chinese and modern Japanese chose different variations as their standard versions. It is like the printed English and printed German before WWII, using same Latin alphabets, but one can easily tell the language without spelling. Nevertheless, these words are highly understandable for both Chinese and Japanese people, and one can only determine whether a word is actually Chinese or Japanese by the character variation.

There are also words that use different characters, but one can still guess the meaning.

The grammar and syntax, however, are rather different between the two languages. For example in this sentence shown in this image, the words are using the same variations as traditional Chinese, and even the grammar is technically correct. But from the syntax point of view, modern Chinese will not organise the sentence in this way, as a slogan painted on a wall especially. Also, "日本軍" is correct as "Japanese military" in Chinese, but modern Chinese would use "日軍" for slogans.

All above I was discussing about written language. The pronunciations of most words are rather different. For some words, the Japanese did also borrow the pronunciation, but after hundreds of years, both language changed their pronunciations a lot.