A question about Idioms

[zoha.zargham]

There is a list of idioms in chapter 8 of the sentence correction.
Does it mean that any other form of those words is wrong and unidiomatic?
For example:
Insist that........Does it mean that "insist on" is wrong?
Claim to be.......Does it mean that "Claim that..." is wrong?
Whether to.......Does it mean saying whether without "to" is wrong?
IS "considered to be" always wrong ?

Many many thanks in advance

[tim]

The best way to approach idioms is to learn some specific correct forms and incorrect forms and go from there. It is dangerous to say that a particular construction is always right or always wrong; remember, the reason these are classified as idioms rather than grammar rules is because someone (the GMAT in this case) made an arbitrary decision which form to use based on what is more prevalent in common usage. However, changing things around in the remainder of the sentence could change which form of the idiom is correct..