Contractions with am, is, and are are commonly
used when the subject is a noun--especially when
the subject means he, she, or it, but there are no
contractions when was or were is used in this way:
Bob is sleeping. ---> Bob's sleeping.
Bob was sleeping. ---> no contraction
María is watching TV. ---> Maria's watching TV.
María was watching TV. ---> no contraction
The bell is ringing. ---> The bell's ringing.
The bell was ringing. ---> no contraction