In Greek every verb is a complete sentence. Its stem is the predicate, its ending the subject.
After Monro, in the sentence [face=SPIonic]e)s-ti[/face], the predicate [face=SPIonic]e)s[/face] means 'is', the subject [face=SPIonic]ti[/face] means 'it' (or 'he', 'she'). Similarly, in the sentence [face=SPIonic]h)/lqo-men[/face], the predicate [face=SPIonic]h)/lqo[/face] means 'came', the subject [face=SPIonic]men[/face] means 'we'.
He then notes that "..the Endings of a Verb may always be translated by Personal Pronouns..". Obviously, 'there' is not a personal pronoun, but it's not much of a stretch from 'it is' to 'there is'.