Qual è la differenza tra je e jij ?Sentiti libero di rispondere con un esempio.

Both are 2nd person singular pronouns. 'Je' is the unstressed variant, 'jij' is the stressed variant.
I've answered this question before, and I have pasted my previous answer with more details and examples below:

Many pronouns in Dutch have a stressed and an unstressed version. Examples unstressed/stressed: ze/zij, we/wij, je/jij, je/jouw.
There are cases in which both forms are correct, but in some cases, only one form makes sense. When you're contrasting, switching who you are talking about, or explicitly asking "Was it .... who ....", you have to use the stressed variant. On the other hand, it often sounds unnatural if you use the stressed version when there is no need. The unstressed versions always take less effort to pronounce for your speech organs, as it typically contains the so-called schwa-sound, which is the vowel you produce by just letting your mouth hang open and letting air through. Like the 'uh' in English 'duh'. I guess it makes sense to be lazy when you can. :-)

Examples:

Heb je de deur dichtgedaan? = Did you close the door?
Basically, you want to know if the door has been closed (by whomever).
Heb jij de deur dichtgedaan? = Did *you* close the door? Was it you who closed the door?
You specifically want to know if the person you are talking to closed the door. You might already have seen the door is closed. Or you don’t know if it’s closed, but are wondering if the door has been closed by the other person, because you did not close it.

Wat vind je? = What do you think? (What’s your opinion?)
Neutral question.
Ik vind (blablabla). Maar wat vind jij? = I think (blablabla). But what do you think?
Because you are switching from what you think to what the other person thinks, it makes sense to stress the pronoun.

Je bent gek. = You're crazy.
Neutral statement.
Hij is gek, maar jij bent nog gekker. = He is crazy, but you are crazier.
You are contrasting two people, thus stressing the subject.

Note that 'je' is also the standard, unstressed form of the direct object, the indirect object and the possesive pronoun. The stressed forms are 'jou' for direct and indirect object, and 'jouw' for the possesive pronoun.

Examples for direct object:
Ik zag je gisteren in de kroeg = I saw you in the bar yesterday
Neutral statement
Ik zag jou gisteren in de kroeg = I saw you in the bar yesterday / It was you I saw in the bar yesterday.
You are emphasizing that you saw the person you are talking to. This makes sense when contrasting: "Ik zag Bob niet, maar jou wel" = I didn't see Bob, but I did see you'

Examples for possesive pronoun:
Is dit je boek? = Is this your book?
You are not doubting that the book or whatever it is is the speaker's, but you might be asking if this is indeed a book (it might be a diary instead for example).
Is dit jouw boek? = Is this your book? Is this book yours?
You are explicitly asking if the person you are talking to is the owner of the book.