Spanish "ti" and "te" and direct object pronoun?

Usted is the formal subject pronoun.

Tú is the informal subject pronoun.

Tu (no accent) means "your."

Subject pronouns are who is doing the action.

Tú eres muy inteligente. - You are very intelligent.

Usted trabaja mucho. - You work a lot.

Le is the formal indirect object pronoun.

Te is the informal indirect object pronoun.

Indirect object pronouns answer the question "to whom?" or "for whom?"

Te quiero (a ti). - I love you.

Le doy la tarea (a Enrique). - I give the homework to Enrique.

(Parts in parentheses are sometimes left off.)

Te is the informal direct object pronoun.

Lo/la is the formal direct object pronoun.

Direct object pronouns answer the question "who? or what?" They are the recipient of the action of the verb.

Te abrazo (a ti). - I hug you.

Lo pego (a él). - I hit him.

Ti is used when it follows a preposition like con, sin, para, por, en. The difference between Spanish tú and ti is a lot like the difference between the English who and whom.

Es un regalo para ti. - It is a gift for you.

Bailo contigo. - I dance with you.

I don't know why the repetition in using "le" and "a él" in the same sentence. Unfortunately, that's just the way it's done!

usted = you (formal)

tú = you (informal)

You are good = Tú eres bueno/Usted es bueno

te = you, to you (pers. pron. 2nd. pers.sing. - object of the verb and ref.)

I love you = te quiero

I'll give you a book = te daré un libro

ti = you, yourself (pers. pron. used as object of prepositions)

he brings it for you = lo trae para ti

you brought it for yourself = lo trajiste para ti

As far as conozco

you can say "lo conozco" without the "usted"

Do you know him? Yes I know him = Si, lo conozco.

In a question (formal) usted - you could say

¿Lo conozco? or

¿Lo conozco a usted? "usted" here even though it is not bad, it is not necessary either.

Only in the case you want to give emphasis you woud use both:

Si, yo lo conozco a usted. = Yes, I do know you.

I hope it is a little more clear now.

All of them mean 'you' in English so it's easier if you have some examples:

TU eres muy inteligente - You are very smart

Yo TE di 10 dólares - I gave you 10 dollars

Pensé en TI - I thought about you