Quick answer
Hard л is very dark, like the l in “full”, with the tongue low and back. Soft ль is bright, like the l in “million”.
l
How to say it
For the hard л, say “full” and hold the l while pulling the back of your tongue down. For the soft one, say “million” and hold the middle.
The pair that proves it
Мел (chalk) and мель (shallows), у́гол (corner) and у́голь (coal). Same letters, different softness, different words.
Why it sounds Russian
The dark hard л is one of the sounds that make a Russian accent instantly recognisable in English — and getting it right does the same for you in Russian.
Typical mistakes
Using the English l everywhere
dark before а, о, у, ы; bright before е, и, ю, я
The English l sits between the two and sounds foreign in both positions.
соль pronounced like “sol”
soften the л
The ь is there for exactly this.
Check yourself
Which word contains Л?