've-' ....I think this is usually 'ver-'
'ver-' can change the meaning of a word eg:
as mentioned in another answer, koop=buy, verkoop=sell
or wag=wait, verwag=expect
or it can change the form of the word eg:
beeld=image (noun), verbeel=imagine(verb)
'be-' can also change the form of a word eg ken=know (verb)
bekend=known (adjective/past participle)
(het geken = knew)
***TAKE NOTE: 'ge-' doesn't always indicate past tense - it can be part of the word eg there are a few nouns starting with 'ge-' eg gedig = poem
Sometimes it is part of a verb in the root form. I can't think of examples, but when you find these, and also verbs that have the prefixes that have been discussed here in your question, THEY DON'T use 'ge-' again in the past tense eg
I buy=ek koop, I bought=ek het gekoop
I sell=ek verkoop, I sold=ek het verkoop - NOT ek het geverkoop