I'm confused about「は」vs「が」 in subordinate clauses. I do appreciate there are inumerable questions on this subject, but to go through all of them hoping to find the exact answer I want would take forever, so I'm hazarding asking one afresh. I hope you don't mind.
Here are a few examples from (very basic) grammar books:
僕が 電話したとき、ナツミは もういなかった。When I called, Natsumi had already left.
弟が 日本に来たら、私は 日光に連れて行きたい。When my brother comes to Japan, I want to take him to Nikkou.
地震は 私が 結婚した年に起こった。The earthquake happened the year that I got married.
東野さんが 薦めてくれた本は 何でしたか。What was the book that Mr. Kyounou recommended?
武が 昨夜見た映画は 楽しかったですか。Did you (Takeshi) enjoy the movie that you saw last night?
私が それを見た場所は そこにあります。The place where I saw it happen is over there.
I am aware that you can't use the same particle in a subordinate clause that you used in the main clause, but how do you know which one should be は, and which one が? Does it simply depend on which part of the sentence you want to emphasize? Does it matter whether the subordinate clause is a noun, adjective or adverb?
If the instances of は and が in the above examples were reversed (each は in place of each が) would this be incorrect, or would it simply change the nuance?
Many thanks in advance!