Spasiba.
As I understood the verb "est''" changes according to the object that follows.
Naprimer:
1. Past: у меня был стакан : In this Case the object is masculine.
2. Future: у меня будут машины : In this Case the the object is in plural.
You're absolutely right!
That's because "стакан", "машины" etc. is grammaticaly the subject of the phrase (like "a glass was", "cars will be").