"Jij" is used when the subject is very important as in it is you and not someone else that is being talked about, otherwise you can use "je". For example, in a sentence "You drink milk." If you say "Je drinkt melk.", I would understand that "You drink milk." but I might focus more on what you are drinking or the fact that you are drinking as opposed to doing something else. With "Jij drinkt melk.", the focus is on "you" and so I am understanding that "You drink milk." as opposed to someone else.
English marks the third person singular with a final -s. Dutch marks it with a final -t, but also marks second person singular the same way. (Except in inverted sentences.)
- I drink. Ik drink.
- You drink. (Early Modern English: Thou drinkest.) Jij drinkt. (Exception: Drink je? Final -t is dropped because of inversion.)
- He/she drinks. (Early Modern English: He/she drinketh.) Hij/zij drinkt.
This difference between English and Dutch is because English has systematically replaced the old second person singular (thou) by the second person plural (you) out of excessive politeness. (Also, in English the infinitive and plural forms nowadays look exactly like the basic singular form: drink. In Dutch they are different: drink/drinken.)
You is never hij or zij. If anyone claims something like this in a Duolingo discussion, they are probably confused by German.
The English language is to blame for some of your difficulties because at some point English speakers perversely started to address everyone in the plural - a practice that was originally reserved for polite forms of address. That's why you address even your little child with you, as if he or she were a pair of twins, and not with thou as people used to do.
Normally, you translates as jij for a single person and as jullie for several. Note that singular and plural also have different verb forms.
- you drink = jij/je drinkt (singular) [think of je as carelessly pronounced jij, similar to a contraction]
- you drink = jullie drinken (plural)
For Shakespeare it would have been simpler:
- thou drinkest = jij/je drinkt
- you/ye drink = jullie drinken
In some situations, people address other people with weird expressions such as "your majesty" or "your honour". Since singular you developed out of a weaker form of this practice, this phenomenon is not as common in English as it is in many other languages. In particular, the corresponding Dutch polite form of address is used not just with royalty and judges but basically with every decently dressed older person until after the exchange of a few words you feel you now know him or her well enough to switch to the familiar mode of address (jij). Of course this is just a usage example. You can also use u to maintain your distance from someone, for example.
The good news about u is that it's the same for singular and plural. The bad news is that the verb forms differ anyway:
- you drink = u drinkt (singular)
- you drink = u drinken (plural)
But it's logical once you know that u developed out of the possessive in phrases similar to your honour and your honours. From this point of view, English works the same way:
- your honour drinks = u drinkt
- your honours drink = u drinken.
- thou = jij (or je for brevity)
- you guys = jullie
- your honour(s) = u
- Dutch is more formal than English, so you can't avoid saying your honour occasionally.