Sandra compra papeles.
Presente simple
Sandra buys papers
Sandra does not buy papers
Does Sandra buy papers?
Presente continuo
Sandra is buying papers
Sandra is not buying papers
Is Sandra buying papers?
Pasado simple
Sandra bought papers
Sandra did not buy papers
Did Sandra buy papers?
Pasado continuo
Sandra was buying papers = Sandra estaba comprando papeles
Sandra was not buying papers
Was Sandra buying papers?