Is there any difference in pronunciation between "monteur" and "menteur"?

[random fulcrum]

yes

with on your tongue is further up in the mouth

and your lips are rounded

[strong pilot]

I feel like giving up. I don't hear any difference between on and en. I can't differentiate them when I hear them. I can make the sounds according to the mouth position but I can't hear the difference even if I am the one making the sound...

[drifting leaf]

"on" is the nasal version of "o"
"en" is the nasal version of "a" (same as "an")

those vowel sound differences are very important to pick up on and will greatly help your understanding and people understanding you, as they're key phonemes of the language

[random fulcrum]

Non askip an c'est [ɑ̃]

Plus loin derrière