Whenever we say a word with /m/, /n/, or /ŋ/ the air comes out of the nose, not the mouth.

We control this air using the soft palate (the soft part on the top of your mouth at the back).

When there is a vowel before a nasal consonant, the soft palate may lower early so that the air partly goes through the nose and partly through the mouth.

In some languages, nasalised vowels change the meaning of a word. Compare the vowel contrasts below. The nasalised vowel is on the right. You can check if the vowel is nasalised by placing your fingertips by your nostrils – if there’s warm air coming out, it means the vowel is nasalised. Otherwise touch the top of your nose and you may feel more vibrations for the nasalised vowel.

French

bonne /bɔn/ “good” – bon /bɔ̃/ “good”

laid /lɛ/ “ugly” – lin /lɛ̃/ “flax” or “linen”

las – /lɑ/ “weary” lent /lɑ̃/ “slow”

Portuguese

vida

[vidɐ] ‘life’ – vinda [vĩdɐ] ‘arrival’

mudo [mudʊ] ‘mute’ – mundo [“mũdʊ] ‘world’

[fɐ] ‘musical note F’ – fan [fɐ̃] ‘fan’

 

Yoruba

(to be rare) dã́   (to polish)

 

English vowels can be nasalised before /m/, /n/, or /ŋ/ – however the nasal quality does not usually change the meaning of the word. In informal speech we could drop the /n/ in can’t and say there is a difference between can’t /kɑ̃t/  and cart /kɑt/

more info in my online course