defining/non-defining clauses

[azz]

a. I like the new method to learn five languages simultaneously.
b. I like their new method to learn five languages simultaneously.

c. I like the new method which allows you to learn five languages simultaneously.
d. I like their new method which allows you to learn five languages simultaneously.

In which case there was/they had another method to learn five languages simultaneously before?

I think in (c) and (d) the clauses are defining. I suppose that would mean that there was/they had at least one more method to learn five languages simultaneously.

In (a) and (b) we don't have a clause. I am not sure, but I think we can't tell if there were other methods before the new one.

Many thanks.

[Alan]

The distinction between defining (restrictive) and non-defining (nonrestrictive) clauses is strictly a matter of punctuation (i.e. written with or without commas).

The suitability of one or the other in any given context depends on sense and meaning, and thus is, in principle, not a grammar issue.