An example taken from the RAE's DPD 5.2b (link):
"...el indefinido 'uno' y su referente es la persona que habla":
Si la ven a una vacilar, enseguida se aprovechan
Can someone please translate this example to English and explain what it means?
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The sentence means something like
If they see one hesitate, they immediately take advantage of one.
Let's see if I can explain it right. The speaker should talk about herself in the first person, and the sentence should be something like this:
Si me ven vacilar, ...
But in this case, the speaker is a third person from the point of view of the subject of the sentence (they). So she assumes herself that third person and thus speaks about herself in that way.
Si la ven...
"If they see her..." Who is her? Oneself. And the word in Spanish to talk about oneself in third person is uno/una. So the sentence is
Si la ven a una vacilar...
The speaker talks about a personal experience, but is probably also advising that it could happen to anyone, so the sentence could also be translated as
If they see you hesitate...
as mdeway says in his comment. He also notes in another comment that speaking about oneself seems old-fashioned in English, but that's not the case in Spanish, at least in this example.