One is from the verb 'ser' and the other from the verb 'estar'. Both mean 'to be'.
yo soy - I am
yo estoy - I am.
What's the difference between them and when should they be used?
2200Lucia and Margarita575756, Many English speakers say they are "fine," "well" or "okay" for the positive feeling, but if someone says, "How are you?" English speakers usually do not say "I am not fine." You are more likely to hear, "I'm not feeling well today," or, "Not too good" (which is INCORRECT English), or some idiomatic phrases, like, "Awful! I caught that cold that's going around!" Using "I am not fine today" would be odd to hear; I don't recommend it.
Lo is a direct object pronoun meaning it, him or you (formal). Siento is the first person singular conjugation of sentir (to feel). The yo is understood from the 'siento'. Normally, 'yo' would be optional, but I believe it would sound weird because 'lo siento' is a common phrase. The literal translation would be: 'I feel it.' De nada = you're welcome or think nothing of it, would literally translate as 'of nothing'.