adjective clauses

[bertg01@hotmail.com]

I would like to know why is not possible to use the relative pronoun "That" in a nonrestrictive clauses. explanations would be appreciated.

[Tdol]

Many rules of language are arbitrary and don't have an explanation. I know of no reason for this, just that we don't do it.

[Casiopea]

I would like to know why is not possible to use the relative pronoun "That" in a nonrestrictive clauses. explanations would be appreciated.

In addition, non-restrictive clauses are set off by two commas. For example, A. and B. carry the same basic meaning. The clause 'which is under the stairs' (in A.) is added information, and not an integral part of the sentence. If we omit the clause (as shown in B.), the basic meaning doesn't change:

A. The blue suitcase, which is under the stairs, is useless.
B. The blue suitcase is useless.

If we use 'that' to head a non-restrictive clause, then we're going to have to use commas to set it off form the sentence proper (as shown in C.), but when we do that, the resulting sentence is rendered awkward (as in shown in D.).

C. A suitcase, that has no handles, is useless
D. ?A suitcase is useless.

Suitcases are not useless, are they?