grammaticality - Question regarding simple/compound/complex sentences

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?

Answer

I think it very likely that your English teacher was giving you ‘baby rules’: the sort of very broad rules you give a child (or learner) to prevent it from hurting itself before it is old enough (or knowledgeable enough) to understand subtle qualifications.

• 
  

  In the case of too, your teacher was probably ‘protecting’ you from a very common learner’s error: using too as a simple intensive, equivalent to very.

  
  

  The ‘grown-up’ rule is that although it is quite true that too almost always involves a comparison of the sort you have in too hot to go out, this does not mean that the comparison must actually be expressed in every sentence in which you use too. The benchmark against which you are measuring may be entirely clear from the context.

  
  

  
  

  Do you want to go for a walk?
  
  No, thanks, it’s too hot.

  
  

  
  


• 
  

  In the case of so, your teacher was probably trying to keep you from using intensive so in an inappropriate register. This use of so is colloquial and almost never found in formal or semi-formal registers such as you are typically called upon to employ in school essays. But in conversation (or dialogue, if you are writing fiction or a play or a script) it’s perfectly acceptable:

  
  

  
  

  It is so hot today!

  
  

  
  

Such rules are pedagogically useful: they keep you focused on what you must learn and keep you from being distracted by all the million-and-one-things you might learn. But in language (as in pretty much everything else), once you reach a certain level of competence you may safely ignore classroom rules and instead follow the practice of writers and speakers you admire.