3. I think this is because he really enjoys teaching Chinese literature — he loves it, in fact!
我想这是因为他确实喜欢教中国文学——事实上,他热爱中国文学！
句中because为连词,连接一个表原因的表语从句,意为“这是因为……”；而当reason作主语时,其表语从句一般只用that引导,不用because。
①“It / This / That+系动词+because从句”意为“这/那是因为……”
He is not here. It is because he is ill.
他没有来是因为他病了。
②“It /This / That+系动词+ why从句”意为“这/那就是……的原因”。
He is ill. This is why he is not here.
他病了，这就是他没有来这的原因。
③“The reason +系动词+that从句”意为“理由是……”
He is not here. The reason is that he is ill.
他没有来，理由是他病了。

4. He's about 28,I think, and is rather good–looking.
我想他大约28岁，长得相当英俊。
I think为插入语。插入语可以插到一个语法结构已完整的句子里去,做一些附加的说明。后接从句要表达否定时,其否定形式应该前移到主句。作插入语可以是不定式短语、现在分词短语、介词短语、形容词、副词、从句等。
常见的插入语有：
I think 我认为
I believe 我相信
I guess 我认为
I suppose 我猜想
I,m afraid 我恐怕
as far as I know / remember 据我所知
you see 你晓得
do you know 你知道吗
do you think 你认为
what's more 此外
that's to say 也就是说
believe it or not 信不信由你
what's worse 使事情更糟糕的是
generally speaking 一般来说
frankly speaking 坦白地说
to tell you the truth 说真的;老实说
to be honest 说实在的
to make matters worse 更糟糕的是
I don't think you are right.
我认为你不对。
I don't suppose I shall be back until 8 o'clock.
我想8点以前我回不来。
I must say, I love that picture of the six horses.
我必须说,我爱有六匹马的那幅画。
By the way, where is Xiao Li?
顺便问一句，小李在哪儿？(介词短语)
Worse still, it began to rain heavily.
更糟糕的是,开始下大雨了。(副词)
I'd really rather not go, if you don't mind.
我确实不想去,如果你不介意的话。(从句)
复杂疑问句/双重疑问句的组成：
疑问词+插入语(do you think / believe / guess)+主语+其他
Who do you think will win the game?
你认为谁会赢得这场比赛？
该句式需要注意：
①插入语为一般疑问句形式;
②插入语后面不再加引导词;
③插入语后接陈述语序。
你认为他在等谁？
Who do you think that he is waiting for? (×)
Who do you think is he waiting for? (×)
Who do you think he is waiting for? (√)