When to use contracted form 'to be'

It depends which words you want to stress: in your examples "late" and "there/ is".

It's not impossible to say something informally like "There it's over there", although here again, the stressed word is "there".

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.



Contractions are an indication of informality, casual friendly speech. Using a noncontracted form, "He is late" could be an expression of anger, rigidity, ... .



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.

I've wondered about this, too. When I researched it, I found that when a sentence ends with a verb, you want to emphasize it. By putting it in contracted form, it is not emphasized, which makes it sound funny. But it only applies to verbs. "I can't" is a totally legitimate sentence. Hope I helped!