As such there is no rule for BE. Atleast I am not aware of one.
As far as SC is concerned, please be very clear of usage of "to be" verbs.
Rule is:
Whenever you encounter "to be" verb, the sentence has to be in parallel.
Other verb forms for "to be"
is, am, was, were, been, being, are.
Consider the following example, taken from manhattan:
The flower bouquet was the husband's giving of love to his wife.
Here, please understand the approach to deal with PARALLEL construction in gmat.
Step 1: Pick the verb.....WAS
Step 2: Note the left side of the verb: "The fower bouquet"
Step 3: Note the right side of the verb: "The husband's giving of love."
Are these two sides parallel? The answer is NO.
Correct form will be: "The flower bouquet was the husband's loving gift to his wife."
Now you see, left side of WAS is a noun, so is right side of WAS.
