/M/, /n/, and /ŋ/ are called “nasal” consonants because to make them, you need to have air escape through your nose. ‘M’ is like ‘b’ or ‘p’ because you make it by putting your lips together (bilabial), but at the same time, your vocal cords are engaged, and air is flowing through your nose. Recall that there is a third nasal, /ŋ/ (ng). It does not become a vowel.
Say ‘mead” [mid] pay attention to the [m] at the beginning. Then say “need” [nid] and pay attention to the [n] at the beginning. Notice where your tongue goes and notice that air is flowing through your nose.
/M/ and /n/ can also be vowels. The way it works is this,
BOT-uhm –> BOD-m[bɑtəm] -> [bɑɾm̩]
If you say the word “bottom” slowly and stress each syllable equally (or listen to a native speaker), you will hear the schwa sound [ə]. If you say it as in natural speech, that sound gets left out, and the flap allophone [ɾ] of /t/ is used instead of the alveolar stop [t].
The dot under the ‘m’ means “‘m’ is being used as a vowel here.”
Here are a few words where ‘m’ and ‘n’ are being used as vowels.
- Rhythm [ɹɪðəm]
- Button [bʌʔn̩]
- Hidden [hɪdn̩]
- Kitten [kʰɪʔn̩]
- Bottom [bɑɾəm]