I can imagine saying, with reference to a card game for example: "The tricky part is learning when to hold and when to discard. " If I substituted "keep" for "hold" it would sound funny: "*The tricky part is knowing when to keep and when to discard." Is that a grammatical difference, or is it simply that that particular idiom does not exist in any known context (known to me) in English? There is not really a grammatical difference between the two verbs "hold" and "keep." I could imagine a part of the English-speaking world where keep would be idiomatic in such a sentence.

Speaking of a stock one might own: "Are you going to hold or sell?"

But: *Are you going to keep or sell.

A proverbial use?: "Those who give, gain; those who keep, lose." That almost sounds grammatical to me, but what do I know. I'm tired and I'm not a linguist.