non-defining relative clause

承接001的回答...第四條他的意思是which作為連接詞時所指的是which字前的那東西，所以你寫的句...this morning which I've found...的which文法上所指是"this morning"而不會是你"想講的那"a book"，所以只能回答"錯句"。

按照你的句子結構次序(假如你堅持要先寫"...was looking for...")，高級一點的寫法是可以寫為
I was looking for a book this morning in which I've found it now.
應該算是解決寫法上的錯誤而算是表達出"你今早找書"的這件事(in which)現時的結果是"I've found it".
不過從老師或嚴格的文學而言，由於in which後是延續講述或詳解前句的情況或細節，後句寫成"I've found it now"是前句的結果可能仍會被人疚病為文理混亂。

第二題我則認為你寫的跟原文要表達的核心用意改變了
原文意思的重心是結束句He is a LAWYER；他住在那裏(澳洲)似是附助背景資料，所以結合重組句時要尊重這重心。你的寫法的重心是你兄弟(是一個律師)今天住在澳洲，而不是你兄弟是(住在澳洲)的一名律師。

一二題的defining relative clause即是指附加形容的片語"是指出那要形容的東西"的獨特性和提供更準確的定位。non-defining relative clause一般是寫一種形容的描述或者只是加插形容前句東西或情景的修飾。
例子：Mary is in high school. She is beautiful.組合句便不能確定Mary是何人或有何'beautiful"特徵以致可以肯定在學校內認出她，這看來便應該是non-defining relative clause.但仍可增加"相對地"使你增加對主角的"感覺"。

(1) It is a defining relative clause because if we dont say 'which we visited last month', then people dont know which museum it is.

(2) It is a defining relative clause because if we dont say 'which was built 300 years ago', then people dont know which house we are talking about.

(3) OK

(4) Not OK! The relative pronoun should be placed right after the noun it stands for. It is not 'this morning' that I have found. It is 'a book' that I have found.