根据张道真与温志达合编《英语语法大全》：插入语可以看作是句子的修饰语，因为它表示整个句子是怎样说的或应该怎样理解。插入语可以是一个插入的陈述句、独立分词、独立不定式或独立副词(短语)。
《英语语法大全》专门用一大节的内容来分析插入语，并按陈述句、独立分词、独立不定式、独立副词（短语）这四类进行了比较系统的分析，同时给出了丰富的例句。从所给例句来看，插入语在句中的位置是比较灵活的，根据情况可以放在句首、句中或句末。至于插入语前后是否需要逗号，大致情况是：原则上要用逗号将插入语与其他成分隔开，但也有不同逗号的。
▲像 I think 之类插入语，通常是需要用逗号与其他成分隔开的。如以下例子均摘自《英语语法大全》：
The boy, I think, has come from the country.
His manners, it is true, are awkward and silly.
Those people, he sometimes felt, were better than us.
This house, I know, will be sold to a rich man.
His wife, I presume, was not listening.
What a strange man, I exclaim, is this fellow!
A very beautiful fairy, so the legend runs, has appeared here.
The people of this country, let it be understood, are poorer now than before.
His knowledge of English, it must be acknowledged, is better than ours.
Let everybody, he ordered, speak out openly.
He is, I am afraid, still very childish.
They are mostly, remember, robbers and pickpockets.
Never, he thought, had they seen such a thing.
It is a bore, you know.
Peptic ulcer, a study shows, usually comes from worries.
It is raining, I think.
What it is all about, I wonder.
He has done everything possible, it seems.
▲但是，如果是在疑问句中，像 do you think, do you guess 这类插入语插在句子中间（这时后面部分要用陈述句语序），则不用逗号将插入语与句子其他部分隔开。如以下例子均摘自《英语语法大全》：
What do you think we should do?
Where did they tell you he was going?
Which do you guess is the better?
How do you say we can settle the problem?

不过，如果不是插在句子中，比如放在句末，也可以使用逗号隔开。如：
When we should build a schoolhouse, did they suggest?
更多相关资料可参考一本书好的语法著作。