Vietnamese learners of English produce the /l/ sound very strangely, sometimes unrecognizable for those who don’t have any experience talking to Vietnamese; this is a well known thing among those foreigners who have experience teaching English in Vietnam. It seems that the lateral consonants are difficult sounds to be produced by Vietnamese learners.
Consonant /l/ is a lateral consonant which means that the air doesn’t go through the mouth in the usual way. Instead of going along the center of the tongue, the air goes along the sides of the tongue. This thing happens because there is a complete closure between the center of the tongue and the part of the roof of the mouth where the contact is to be made. In the case of /l/ sound the contact is made between the center of the tongue and the alveolar ridge.
If you try to produce a long sequence of alternation between /d/ and /l/ without any vowel in between (dldldldldldl) then you will be able to feel the movement of the sides of the tongue when producing /l/.
Note: It is very important not to move your tongue at all.
The /l/ sound can be found in initial, medial, and final position. However, it has one unusual characteristic: the consonant /l/ before vowels, sounds different than the /l/ found in other contexts. In order to exemplify the above statement let’s take a look at the following two words:
Lea /li/ (often pronounced /leɪ/)
In the word "eel", the /l/ sound is produced with the back of the tongue raised and it has a quality similar to an /u/ vowel. It is called a "dark l". The sound /l/ in the word "lea" is produced with the front of the tongue raised and it has a quality similar to the vowel /i/. It is called a "clear l".
The "dark l" is found when the /l/ sound is placed before a consonant (as in the case of the word "eel"). We can say that the "clear l" will never occur before consonants (or before a pause) but only before vowels, and the "dark l" never occurs before vowels.
As a conclusion we can say that the "clear l" and the "dark l" are allophones of the phoneme /l/ in complementary distribution.
Another often found allophone of /l/ is when it follows /p/ or /k/ at the beginning of a stressed syllable. In this situation, the /l/ sound is produced without the voicing found in most realization of this phoneme (it is devoiced).