Why te quiero not tu quiero or ustedes quiero?

Answer 1

When you are trying to say "I love you" you conjugate querer in the yo form because you are the one doing the loving. Then you want to answer the question of who or what do I love? Therefore, you use a direct object pronoun to answer this question. Since te is the direct object pronoun for the tú form, that is what you use.

Answer 2

This is a grammar question. Tu and Uds. are subject pronouns. Te and los are the corresponding direct object pronouns. In English, "you" is both a subject pronoun and a direct object pronoun. This makes the direct translation confusing from a grammar perspective, because it is not immediately obvious that the "you" in "I love you" is the direct object pronoun and not the subject pronoun. To make it easier to see, let us replace "you" with "he" and "him", where "he" is the subject pronoun and "him" is the direct object pronoun. The proper sentence is "I love him", not "I love he".

Answer 3

Briefly, "tu" (with an accent sign) is used as a subject; "te" is used as an object (both direct and indirect object).