Good remark! You is the second-person personal pronoun, both singular and plural; that is why were is used. Regarding the above verb forms, one should simply memorize them. In conditionals, he/she/it with "were"is also used.
Apart from etymology, there are also some rules that have to be memorized (as I mentioned above).
"Basic Rule: A linking verb links the subject with a noun, a pronoun, or adjective. A linking verb must agree with its subject in number - plural or singular. If the subject is singular, the linking verb must be singular. If the subject is plural, the linking verb must be plural. Four