我一写完日记就上床睡觉。 this sentence is the grammar I mentioned in my comment, using  一⋯⋯就⋯⋯ to show something is right after another.  In this case I would say an english translation could be: I go to bed right after I write in my journal. 


下课了我就去看电影。The 了 is a separate grammar point here. 下课 is a verb to get out of class (cause its over) and 了 is serving its common purpose after a verb to show completion.  The 就 here means as soon as.  So you could translate the sentence: As soon as I am out of class I go to see the movie.  

So the two formats you mentioned aren't directly related, and it may not be useful to compare them too closely.  If we were to compare them anyway, I would say the first sentence was worded to show "do action 1 then immediately action 2".  The second sentence was worded to show "at that time immediately do action 1".  

 link to a wiki page on the 一⋯⋯就⋯⋯ structure for you.  The same page should have links to a variety of other 就 uses as well: 

https://resources.allsetlearning.com/chinese/grammar/Events_in_quick_succession_with_%22yi..._jiu...%22