1. start to do sth. = start doing sth.
一般情况下,两者可以互换.
但在下列三种情况下,只能用start to do sth.
(1)当主语是物而不是人时.
如：The ice started to melt.
冰开始融化.
（2）当start用于进行时态时.
如:She is starting to cook the dinner.
她开始做饭
（3）当start后面的非谓语动词指心理状态或精神活动（即表示想法、
意见等的词,如think, realize, wonder, understand等）时.
如：She started to wonder who had done it.
她开始想是谁做了这件事.
2. ask sb. to do sth.
3. 干完某事
4.& 5. 用单数
6. Russian
7. 非谓语动词(动名语、不定式）作主语时,谓语用三单形式.