he buscado = I have searched (looked for) le he buscado = I have searched (looked for) him/her/you/it Estoy siento = incorrect, we don't say that in spanish. "Estoy" means that you are in a place or in a special mood, like "Estoy en la universidad" (I am in the university), or "Estoy feliz" (I am happy), but to feel sorry is not that kind of feeling. Lo siento = I'm sorry. The verb is "sentir" (to feel) but in this case is not that meaning, but "to feel sorry", and the "Lo" is hard to explain, but I will try: the complete sentence would be "Yo lo siento", meaning "I am sorry (about something)", the "lo" indicates that "something", and then you drop "yo" and get "lo siento". Maybe there is a better explanaition.

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"Le" is the object, it means either "he", "she" or "it". "Le he buscado" as a whole phrase can mean "I've tried a lot of different things to solve it" or "I've looked for a lot of different answers." Siento ≠ sorry. Siento comes from sentir ("to feel"), "Lo siento" = "I feel it", there's another phrase that may help you understand it: "Siento mucha pena por ello" which literally means "I feel a lot of sorrow for it/about it". "I'm sorry" could also be "Estoy apenado" or "Estoy arrepentido" (use the latter one only if you have done something really bad)

Good question Ahmad. "He buscado" means "I have searched". The "le" in front is what was searched for; it is the direct object pronoun meaning "él". I have searched for ___. "Le" is replaceable by "lo". "Lo" is the standard direct object pronoun whereas "le" is only commonly used in certain dialects. It is actually used like this in formal Spanish: "I throw the ball to him" => "Le" tiro la bola : "le" (here meaning "him") is an indirect object pronoun which signifies either él or ella. And the ball which "directly receives" my action "to throw" plays the role of a "direct object pronoun". That said, "lo" in the second sentence is a direct object which means "it". And "Lo siento" means "I feel it", usually 'I feel your sorrow." thus "I am sorry." Hope this answers your question.