a single completed action?

Hello everyone,
I am reading bbc.co.uk/worldservice/learn … v106.shtml
I wonder what is the menaing of ‘a single completed action in a defining relative clause’, especially the phrase 'a single completed action’?

Thanks in advance.

This is that text:

[i]Note that when we are talking about a single completed action in a defining relative clause, we cannot use an active participle:

The girl who fell down the cliff broke her leg.
(NOT: The girl falling down cliff…)[/i]

The point of ‘a single completed action’ is that the action is finished and would appear in a past tense (‘who fell down’). We cannot use the ‘-ing’ participial clause in that case; it can be used only as a present form (‘Lawyers [who are] acting for Biggs have said…’).

Hi,

I assume you are referring to this:

In this sentence ‘fell’ is the single completed action and refers to the one finished action in the past or the past simple. It is a defining relative clause because it describes that girl in particular.

Alan