@KvdPeijl,
Quote:but as an ESL teacher I am looking for the rules for that, so I can help my students understand.
Don't focus so narrowly on the "rules". Just do a lot of actual practice using the be verb. Students do not learn a language from memorizing rules.
Quote:For example:
"He's late."
Contractions are an indication of informality, casual friendly speech. Using a noncontracted form, "He is late" could be an expression of anger, rigidity, ... .
Quote:But not:
"There he's."
The reason that, a contraction, doesn't fly in the above case is because fronting the adverbial 'there' is an emphatic - "There he is!"
The non-emphatic, "He's there", with a contraction is fine.