there are no rules, you just have to learn which word uses which one. the only rule is all plurals use de for example het meisje: the girl de meisjes: the girls
examples of words that use de: vrouw (woman) ~ appel (apple) ~ melk (milk)
words that use het: meisje (girl) ~ kind (child) ~ sap (juice)
is lezen also past tense? like would you use it to say "i have read that book (in the past)"?
as someone who's only started a couple days ago (and so not good) would "Ik lezen Harry Potter" be a decent translation to "I read Harry Potter" or am i missing something
The paste tense of 'lezen' is 'lazen' (1st person plural). The correct translation of "I read Harry Potter" is "ik lees Harry Potter" ('ik' is 1st person singular, so the verb is also 1st person singular).