My favorite question! 1) When ㄹ occurs in the beginning of a word, the standard pronunciation is supposed to be L. For example, the Korean word for "Russia" = 러시아 (written 로씨야 in North Korea) is pronounced with an L-sound by newscasters but many speakers (myself included) generally pronounce it with a lightly trilled R. 2) If ㄹ occurs in the middle of a word and the syllable before has a vowel -- for example, the Korean word for "wind" or 바람, it should always be the lightly trilled R sound, never the L sound. 3) If ㄹ doubles (or ㄹㄹ), then it should always be the L sound -- for example, 말리다 ("to dry" or "to break a fight", depending on the situation) -- should always be pronounced with an L sound. If you have any other questions, send me some words with ㄹ, and I'd be more than happy to teach you which sound you're supposed to use. :)

It doesn't in either case. "ㄹ" is a new phoneme that you need to learn. The closest approximants in English are L and R, but the reality is that it's neither sound, and substituting L/R for ㄹ is a pronunciation error.

Never. ㄹ only sounds like ㄹ, always.

When the consonant ㄹ is used as init sound, it can be pronounced either R or L as in English. 1. When it follows a letter that ends in a vowel, it is pronounced as in R. 아리랑 [a-ri-rang] 우렁[woo-reong] 2. When it follows a letter than ends in 받침, it is pronounved as in L 한라산 [할라hal-la-san] Therefore, the noun 라디오 can be pronounced by either [la-di-o] or [ra-di-o] in two ways. If you want to pronounced it in much more similar to 'L' or 'R' as in English, just add the vowel 을 or 으 to the beginning of the word. For instance, [(을)럭키], lucky [(으)럭키], rocky When the consonant ㄹ is used as final sound, it is pronounced as in L-pronunciation. 한글 [한글, han-geul]

Just practice listening and speaking with feedback. And don't use Romanization.