relative clauses

[skyway]

Hi , could you please help me understand these:
1." i spoke to a woman on the phone.She told me to call back later =
The woman who I spoke to on the phone told me to call back later"
Can we say "a woman who i spoke........" instead of " The woman who i..." ?
if can't why??????? 2.Are these defining or non-defining relative clauses? Why?????: "The mall where i sometimes buy clothes has fabulous shops" "The computer games which i played yesterday for the firsttime are really great" "The cheque which i deposited on the 10th was for 450 pounds" thanx

[Alan]

(ONE question per post, please - see website guidelines!)

Yes, you certainly can construct a defining (a.k.a. restrictive) relative clause equally well with an indefinite as with a definite article. In many cases - such as this - it would make no significant difference (beyond a possible implication that I spoke to one or more other women on the phone, not just this one.)