Translation:Between me and you, I'm eating all the chocolate.
Yes, it's because of the ל. The same happens with פ and כ who at the beginning of the word are pronounced p and k respectively, but become f and kh if ל or ב or ו or כ is written in front of them, but only if the word is indefinite. If it's definite, the pronunciation doesn't change.
However, this is a rule not followed in everyday language, but rather in formal language.
In the book the Elements of Style by Strunk and White, we read on p. 12: “between you and me” is correct. Strunk and White’s example to illustrate this point is “Let’s talk it over between us, then, you and me”. Other grammarians concur that you put yourself last when “me” is part of a compound object.
Now we know what grammarians agree on. However I would say that in colloquial speech, “between me and you” is as okay to say as “it’s me”. In grammar books, “it’s I” and “between you and me” are the more correct forms though.
"It's I" vs. "It's me" is a question of grammar. Is the preference to "between you and me" over "between me and you" similarly a question of grammar, or just of manners - that it's more polite to put "me" last? I was educated not to say "אני, רבקה ויוסף but rather רבקה, יוסף ואני - put myself last; as a matter of manners and not grammar.
BTW, even taking manners into account I don't see an a preference between ביני לבינך and בינך לביני.
(This talking of preference between X and Y where X and Y are themselves "between W and Z" boggles my mind! (-: )
You say that “It’s I” vs. “It’s me” is a question of grammar, and that implies that saying “it’s me” is grammatically incorrect. However, the way grammar works is that if enough people start using something that was formerly considered incorrect, it often becomes accepted and therefore “correct”. 400 years ago, you were incorrect to say “you” when addressing one person; the proper thing to say in that case was “thou”. However, enough people started ignoring the rule so that the rule disappeared from most people’s minds to the point that you could say “it’s correct to say ‘you’ when addressing one person.” But between the time that a person might be thought stupid to say “you” instead of “thou” when talking to one person and “who would ever think it’s bad to say ‘you’ to refer to one person?” is where the arguments flare up about what is correct and what is incorrect.
So your question of Is the preference to “between you and me” over “between me and you” similarly a question of grammar, or just manners? cannot be answered without debate, because at this point in our history, it’s still considered grammatically incorrect by some and acceptable to others to say “between me and you”.
Apparently we are all wrong. This isn't a phrase I use (normally I'd say just between the two of us, or don't tell anyone...) I wonder if it's because it goes against what we normally think of when to use I vs me?
Oxford dictionary: In standard English, it's grammatically correct to say 'between you and me' and incorrect to say 'between you and I'. The reason for this is that a preposition such as between should be followed by an objective pronoun (such as me, him, her, and us) rather than a subjective pronoun (such as I, he, she, and we).
The short answer is that the speaker is using the subjective pronoun I after a preposition, rather than the objective me, and modern English grammar dictates that pronouns that follow a preposition such as between should be objective ones (me, you, us, him/her, it, them).
Because I'm in the app and therefore can't edit: http://www.quickanddirtytips.com/education/grammar/between-you-and-me?page=1
The flame roared above me, the dog chased after me, the woman walked with me. And if you're with someone, the flame roared above him and me, the dog chased after him and me, and the woman walked with him and me. Personal pronouns after prepositions are not in a subject form, so it’s not between you and I, it’s between you and me. Is it “He gave a gift to Yossi and I”? Take out the “Yossi and” and you will know that “He gave a gift to me” is correct.
DanielAlpe, I assume your last sentence meant to say: “Between you and I… is proper when the subject of the sentence is you and I.” The meaning of the Duolingo sentence is “I’m eating all the chocolate, and this information is between me and you.” “I” is the subject of the sentence because I am the one who is eating all the chocolate. The first person singular pronoun in the phrase “Between me and you” is not the subject of the sentence.
I did look it up. It's the other way round from what you wrote. The correct way of saying it is "Between you and me" (because "I" cannot come after a preposition), but people started using "between you and I" more, and it's becoming common.