We read about these three classes of sentences. For example,
- It is too hot to go out without an umbrella. (Simple)
- It is so hot that we cannot go out without an umbrella. (Complex)
- It is very hot and so we cannot go out without an umbrella. (Compound)
My teacher then (around a decade back) pointed out the "too-to" structure in simple sentences and a "so-that" structure in complex sentences. He added that casual sentences like I am so busy or I am too tired are wrong and in each of those cases, we need a very, as in, I am very busy or I am very tired. His logic was that so or to need a that or to for meaningful completion, going by the examples above.
Is he correct? Does modern English deem I am so busy to be suitable or valid in formal contexts (say in written English or the English taught at schools)? Is It is too/so hot and so we cannot go out without an umbrella a valid compound sentence?