When saying the f sound, your front teeth should touch your lower lip. D is unaspirated, while t is aspirated. To produce d , refrain from expelling a puff of air. To produce t , expel a strong puff of air. It is produced by pressing the tip of the tongue against the upper teethridge and letting air come out through the nose.
L is produced at almost the same position as n , so some people confuse the two sounds.
The position where l is produced is actually a little behind that of n , and when the sound is spoken, air comes out by the sides of the tongue. To practice the difference between these two sounds, concentrate on whether air comes out of your nose when you say them. Hold your nose shut, and say n . If you feel that air is being blocked in your nose as you do this, your saying the sound correctly. The position and manner in which they are produced are the same.