Please follow the following transformations: a) Tu conosci il fatto che io non sento bene ? il fatto ==> lo (object pronoun) conoscere ==> sapere b) Tu lo sai che io non sento bene ? Now: drop the subject pronouns, as they are impl;icit from the verbal voice c) lo sai che non sento bene ? Turn on a global object for /sentire/ meaning a physical condition related to the subject; in this case /ci/ is a substitude for a /global context/ related to /to hear/. d) lo sai che non CI sento bene ? See the difference with a specialized object: e1) lo sai che non sento bene il suono del telefono ? e2) lo sai che non sento bene la musica ? (e1) andd (e2) do not speak about an intrinsic condition of the subject, related to the action of /to hear/. (c) is also ok, but (d) is better! remember: /sentire/ has two tanslations with DIFFERENT meanings: to hear to feel (obviously here is /to hear/)

In that sentence "lo" is a personal pronoun (III p. sing.), it means "it" and it has a neutral values. Its aim is to change a question in a rhetorical one. "Sai che non ci sento bene?" ("Do you know that I haven't good hearing?") "Really? I didn't know." :-O "Lo sai che non ci sento bene?" (literally: "Do you know it that I haven't good hearing?" with the mean of: "I should already know that I haven't good hearing, shouldn't you?) "Sorry! I'll speak louder." :-/