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.
b. You are very beautiful.
Above, each clause has a subject-verb pair; two subjects and two verbs.
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.
b. A beautiful girl was sitting under the tree.
Above, each clause has a subject-verb pair: there are two subjects and two verbs.
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 so
Verb: surprises
Subject: you
Verb: should
Sentence 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:
Here's how it breaks down:
Subject: (The fact)
that you should say so Verb: surprises Object: me
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.
(Adverb clause)
2. I saw a beautiful girl who was sitting under the tree. (Adjective clause)
3. That you should say so surprises me. (Noun clause)