Summary:
ཡིན་: [context] [object] ཡིན་
meaning: “to be” when talking about what the speaker has personally experienced.
རེད་: [context] [object] རེད་
meaning: “to be” when talking about general statements, both about the speaker and about other things.
Known components can be omitted in sentences, just like subjects are turned into personal pronouns in English.
Fun fact:
“Yes” is actually the sentence “(what we are talking about) is/རེད་ (what you are telling me)” trimmed down to its essential component: the verb.