Translation:You are a fast boy.
What's the difference between "Je" and "Jij"?
It's explained here: https://www.duolingo.com/comment/3734337
Jij is Je with emphasis (on the pronoun).
When do i use snel and when do i use snelle?
You use snel in situations like this: "De slak is snel / Het paard is snel". (slak = snail / paard = horse)
You also use snel when it's a 'het' word with a indefinite article:
De snelle slak - Het snelle paard
Een snelle slak - Een snel paard
Is there a rule to add the 'e', because I didn't understand.
You use it when it's in front of an 'het' word (het meisje) and has an indefinite article (a/een).
Het snelle meisje - Een snel meisje.
What about 'de' does the word after de gets 'e' (de snelle or de snel)?
De words always get snelle in front of them.
when do you use bent and when do you use zijn?
"Bent" is used for je/jij (you - singular).
"Zijn" is used for wij/jullie/zij (we/you-plural-they).
I almost heard "Je bent een smelly jongen"
The word "kort" is also listed as having "quick" as a possible translation. When to use kort as opposed to snel?
I suppose they're thinking of contexts like 'a quick chat' - 'een kort gesprek'. Snel = fast.
It bodes well for me that speed impresses you
I can't tell you how many times I go on auto pilot and see Je and respond I like French ! Arrg