I've already understood, and you can't say which is main clause and which is subordinate clause in the third sentence. That is to say, you dare not say "the defination of a complex sentence is worng in this case".Whitemoon, the main clauses are underlined and the subordinate clauses are in bold.
1. I love you because you are very beautiful.
=> Notice that if we omit because we are left with two complete sentences:
a. I love you.Above, each clause has a subject-verb pair; two subjects and two verbs.
b. You are very beautiful.
2. I saw a beautiful girl who was sitting under the tree.
=> Notice that who replaces a beautiful girl, and so by replacement we can get two complete sentences:
a. I saw a beautiful girl.Above, each clause has a subject-verb pair: there are two subjects and two verbs.
b. A beautiful girl was sitting under the tree.
The following example sentence also houses two subject-verb pairs, but one of the subject-verb pairs is embedded in the subject of the other subject-verb pair, which means this is and cannot be a complex sentence:
3. That you should say so surprises me.
Subject: That you should say soSentence 3. is an example of a simplex sentence (SVO) that happens to house a clause as its subject. Now, if it were complex, we would be able to get two complete sentences out of it, wouldn't we? But we can't:
a. You should say so. <a complete clause>Here's how it breaks down:
b. *Surpises me. <fragment: no subject>
Subject: (The fact)that you should say so
The subject is an elliptical noun clause, the head of which; i.e., the fact, is elided, but its relative that is overt.
To diagram that sentence,
IP branches into IP (that you should say that) and VP (surprises me)
VP branches intoV (surprises) and NP (me)
If you have an linguistic argument to show that example 3 is complex, I'd like to see it. As it stands, your subordinating clause is a subject, nothing more:
1. I love you because you are very beautiful. <FUNCTION: conjunction>(Adverb clause)
2. I saw a beautiful girl who was sitting under the tree. <FUNCTION: modification>(Adjective clause)
3. That you should say so surprises me. <FUNCTION: subject>(Noun clause)
Thank you very much!
May you be happy, healthy and wealthy!