That's a bit harder to explain as there is not really a perfect way to translate, In time sentences like that is more about the feeling 才 and 就 carry. 才 means something is not early enough, most often is said as 'until/till' 他8点10分才来, he didn't come until/till ten past 8. 就 means something is early enough.他7点50分就来了.He came/arrived at 7.50 (don't translate 就 here)