Englishfor English speakers
upon
Preposition
—
on or onto
The police were called upon to save him.
The snow fell gently upon the lake.
—
going to happen
Christmas is almost upon us.
—
at (a time)
Upon his death, she became the owner of the store.
their
Pronoun
—
If something belongs to them it is their thing.
That's not their car, it's ours.