We don't have 'et' in English, of course. We use it in Hebrew in front of a definite direct object. That is, a word with 'the' that a subject is doing something to. ex: Annie eats the apple -- Annie eats 'et' the apple. || 'et' comes before 'the apple' to let us know that the apple is being acted upon (being eaten) by the subject (Annie). We don't put 'et' in front of Annie because she is the subject, not direct object. We also would not put it in front of 'an apple' because 'an' (not that an equivalent word exists in Hebrew, but the concept exists, of course) denotes an indefinite [direct] object.
Thank you so much! This really makes "et" make so much more sense, תודה!