We use 'is' and 'was' for singular subjects.
Ex.
Annie is good.
Annie was good.
However, 'was' should be used when referring to the past and 'is' should be used when referring to the present.
'Are' and 'were' should be used for plural subjects. 'Are' is used in the present time while 'were' is used when referring to the past. There are special cases though.
One special case I could think of is using the pronoun 'you'.
Ex.
You is charming.
*The subject might be singular, but this is a special case and should follow a plural verb so the right way is:
You are charming.
Hope this helps(:
