In MGMAT, Chap 4, it mentioned that two sides of a linking verb (is, was etc) should be parallel.
i.e. The charity work is a doing of good deed. Wrong because charity (noun) is not parallel to a doing (participle).
Instead, it should be The charity work is an act of good deed. charity (noun) is now parallel to an act (noun).
But what about this sentence "The boy is cute" Here we have a noun (the boy) and an adjective (cute) on two sides of the linking verb (is).
How do we know when a linking verb, such as "is" requires the use of parallelism (as in the case of the first example)?
In 2nd example, "Cute" is a Noun but in this case is working as a Adjective, and thus both sides are parallel.
As far as your 2nd query is concerned, Whenever helping verb is used as Linking verb, both sides must be logically parallel.
Brush up Linking verbs concepts in detail.
Hope it helps
If you like my Question/Explanation or the contribution, Kindly appreciate by pressing KUDOS.
Kudos always maximizes GMATCLUB worth -Game Theory
If you have any question regarding my post, kindly pm me or else I won't be able to reply