"You read the newspapers."
Translation:Du läser tidningarna.
Ni is the plural form of du, kind of like the Southern American English "you all" or "y'all." It is used when you are addressing multiple people. Ex: (if you were talking three friends) Har ni böcker (Do you have books?) It is ni here because you are talking to more than one person.
- -en is the singular definite suffix: tidningen = the newspaper
- -ar is the plural suffix: tidningar = newspapers
- -na is the plural definite suffix: tidningarna = the newspapers
The pattern depends largely on the verb's declination. This page has some more info on that: https://www.lysator.liu.se/language/Languages/Swedish/Grammar.html#nouns
No, that's a common myth. You should use du when talking to one person, and ni when talking to more than one person. Swedish never used ni broadly as a formal pronoun, and you may even offend some older people if you use it, so while it does occur rarely, this is largely misinformed and not advisable.