I don't hang around so much anymore, but every now and then it's fun to pop in just to see what everyone's talking about these days. Sometimes it serves to keep the old brain waves pumping.
Hereinchina, your question is not really about grammar if you were expecting to find some sort of "rule" that would force the selection of one of your alternatives as opposed to the other. In your example, both "can" and "could" are grammatically correct, as Lorikeet has confirmed.
What you need to keep in mind is that language (any language) is formed by a speaker/writer to express a particular meaning, and that meaning, at the moment of forming, is precisely known only to the speaker/writer. So (s)he chooses the forms that best fit the meaning intended. The general point is,
"If I won the lottery, I would quit my job faster than you can say Jackie Robinson."
"If I won the lottery, I would quit my job faster than you could say Jackie Robinson."
...are both perfectly grammatical sentences, but they have slightly different meanings. The choice would be made by the speaker/writer according to what is in his or her mind at the precise moment of speaking or writing. So how would that choice be made?
are, of course, related and both express the concept of "possibility." You could paraphrase either of your sentences as, "If I won...faster than it is possible
for you to say, "Jackie Robinson."" The problem for the speaker (and, incidentally, also for the hearer, since the hearer is tasked with interpreting the real and precise meaning of the speaker) is that there are several 'kinds' of possibility:
"Can you speak English?"...refers to ability.
"You can't smoke here." ...it's against the rules.
"Can you pass the salt?" ...request (Is it possible for you to...)
There are more 'kinds', but these will do to illustrate the point. It would be also "correct" for someone in particular speaking circumstances to say:
"Could you speak English?" ...here the speaker might be referring to a particular time in the past that the hearer might have been talking about in a story. It could mean: "At that time, was it possible for you to speak English?" In addition, in another different usage situation, it might also be a polite request for someone to please use English.
"You couldn't smoke here."...again this might be a reference to an earlier time when smoking was forbidden.
"Could you pass the salt?" ...a perfectly good request, as was the other with "can," but this one is seen as "more polite." But when is politeness required? It is when your relationship with the hearer is less familiar, or more remote. "Can you pass the salt?" is perfectly good when you are speaking to your sister around the family table. But when you are a guest at another family's table, "Could you pass the salt?" would be more common because it acknowledges the remote nature of the relationship between speaker and hearer.
The general point is, "use can
when there is no requirement for any sort of remoteness; use could
when remoteness (of some sort) is needed or useful.
In your examples, I would explain it this way:
"If...than you can say Jackie Robinson.
" means "than it is possible for you, now, to say Jackie..."
"If...than you could say Jackie Robinson.
" means "than it is remotely possible for you ever, no matter how long you might practice, to say Jackie..."
Forgive me for such a long lesson on a seemingly simple question, Hereinchina, but your question actually is not so simple. "Remoteness" is a deep principle in the English language, and awareness of it ables you to make many choices between two alternatives in English with ease. Google "Michael Lewis" on your computer, and check out his delightful and very helpful book, The English Verb