How Thais (and Asians) pronounce their "R"s and "L"s
In the European languages like English, German, Italian, French and Spanish, the sounds for "L' and "R" are quite distinct. This is because the position of the tongue is either at the extreme front end of the mouth (pushing quite forcefully against the top teeth) for the "L" or at the general back part of the mouth for the "R". The French "R" is with the back of the tongue almost being swallowed at the back of the mouth, while the German "R" is vibrated very vigorously in the middle of the mouth. The English "R" is produced with the tongue floating just beneath the palate towards the back of the mouth.
So the "L" is a very different animal from the "R" because the "L" is a strong tongue-pushing-at-the-front sound and the "R" is a vibrating or rolling tongue sound nearer the back.
However Thas (and Asians in general) produce an "L" by positioning the tongue very lightly against the palate well behind the teeth just in front of the middle of the mouth.
While the "R" is in the same position but with the tongue allowed to drop a little so that it "floats" ever so slightly below the palate.
The Asian "R" becomes an "L" simply by touching the palate with the tongue.
That's why L and R sound so similar to our Western ears - because they are very similar. And when Thais (and Asians) speak a little lazily, or fast - which is usually the case - when pronouncing "R" it's usually easier to let the tongue touch the palate instead of leaving it "float" just below it - which results in an (Asian) "L".
At the end of a syllable, Ls and Rs aren't fully enunciated. Instead you simply close down your palate and push your tongue down. The resulting sound is very like our English "N".
That's why อาหาร and บิล are pronounced "ahaan" and "bin", respectively (not "ahaar" and "bill").