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Why do we say
but
?
Aren't these equal in terms of grammar? If so, why with "tú" in both cases it's "te", but with "usted" it's "le" and "se"? How do I know when to use "se" and when "le" with "usted"?
First, let's make a brief clarification: usted, in Spanish, is always gramatically treated as a third person (the same as él, ella, ellos, ellas).
Now, to your question. Indeed, te, se, and le are all personal pronouns. They're a special type of personal pronoun that we call pronómbres personales átonos. They can basically play two roles: as the verb complement (direct or indirect), or as the reflexive particle that you put before or after a verb.
Let's look at this useful table of pronómbres personales átonos:
As you may note, for the first, and second person pronouns (yo, tú, nosotros, ustedes), there are only distinctions between singular and plural cases. However, in the third person pronouns (usted, él, ella, ellos, ellas), there are distinctions regarding number (sing. or plur.), gender (m. or f.), and syntactic function (direct obj., indir. obj., or reflexive).
In the first set of sentences that you put as an example, le and te are being used as the indirect object:
A ti: 2nd person singular = teA usted: 3rd person singular as indirect object = leIn the second pair of examples, le and se are being used as reflexive particles of the verb preocuparse.
Tú: 2nd person singular = teUsted: 3rd person as reflexive = se
