Anonymous

“Hi. Would you say the only difference for these is that of formal and informal usage?

They speak better than them/they do.

They spoke better than them/they did.

”

I would consider "them" in both sentences to be very informal, even comical for most people who have been taught basic grammar.

This is the definition and an example sentence for the word "syntax" in the Naver.com (online) dictionary.

Syntax is the ways that words can be put together, or are put together, in order to make sentences.[ TECHNICAL ] 

   His grammar and syntax, both in oral and written expression, were much better than the average.

Could we have written the example sentence like this?

His grammar and syntax, both in oral and written expression, were much better than the average was.

”

It sounds okay, but I would change "the average" to "average" or "the average person's (grammar)."

Similar to this? Would you say what is in parentheses is optional?

He is taller than he (is).

He was taller than he (was).

”

These sentences are correct, and you can omit the words in parentheses.

Sorry, is this correct, too (without the word "did")?

They spoke/played better than they.  

”

This is okay. However, with all these sentences, you are using the same pronouns to refer to two different nouns, which is confusing. It would be less confusing to have "he is taller than she" or "she was faster than it", since each pronoun suggests a specific group of nouns. When you use "they are taller than they", you will need to clearly explain the difference between the two "theys" in the sentence.