Form and rules
How it works
Existential there introduces new information: There are several options; There seems to be a problem.
There can combine with perfect/modals: There has been a delay; There may be another explanation.
Preparatory it can introduce an adjective + infinitive/that-clause: It's important to check; It's surprising that nobody called.
It can also be used in cleft-like anticipatory structures with clauses: I find it difficult to concentrate.
Choose there for existence/appearance; choose it for evaluation, weather/time/distance or as a placeholder for a later clause.