先观察原句
后自主感悟
①What it was to become was uncertain until between 4.5 and 3.8 billion years ago when the dust settled into a solid globe.
②The earth became so violent that it was not clear whether the shape would last or not.
③What is even more important is that as the earth cooled down，water began to appear on its surface.
④It was not immediately obvious that water was to be fundamental to the development of life.
⑤What many scientists believe is that the continued presence of water allowed the earth to dissolve harmful gases and acids into the oceans and seas.
⑥So whether life will continue on the earth for millions of years to come will depend on whether this problem can be solved.
1.以上各句皆为主从复合句，句中黑体部分为名词性从句作主语，称之为主语从句。
2.what引导主语从句，what在从句中作表语(句①)，主语(句③)，及宾语(句⑤)。
3.whether引导主语从句，意为：是否(句②⑥)。
4.可用it作形式主语，真正的主语为名词性从句(句④由that引导主语从句)。
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