Expression: Stop me if I've told you this before

[johny D]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #119 "Gerund or Infinitive", question 1

......... me if I've told you this before.

(a) Stopping
(b) Stopped
(c) Stops
(d) Stop

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #119 "Gerund or Infinitive", answer 1

Stop me if I've told you this before.

Correct answer: (d) Stop
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What does it mean?
I so don't get.
Can you please help me?
Thank you!!!!
johny

[Alan]

Hi johny,

This expression: Stop me if I've told you this before is used in conversation by someone who is about to tell a story or report an incident. They are asking the audience (the people they're talking to/or the person) to tell them if they already know the story in which case they won't repeat it.

Alan
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English as a Second Language
You can read my ESL story Party Conferences

[Arpadarpa]

Alan,
Why present perfect was used? I had used past perfect if I would have written this.