Nasalization
When a syllable ends with m or n, this consonant is not fully pronounced, but merely indicates the nasalization of the vowel which precedes it. At the end of words, this sometimes produces a nasal diphthong.
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Monophthongs Diphthongs Grapheme Pronunciation Grapheme Pronunciation -an, -am, -ân, -âm1 /ɐ̃/ -am2 /ɐ̃ũ/ -en, -em, -ên, -êm1 /ẽ/ -em, -ém2 /ɐ̃ĩ ~ ẽĩ/ -in, -im, -ín, -ím3 /ĩ/ -en-, -én-4 -on, -om, -ôn, -ôm3 /õ/ -êm2 /ɐ̃ĩ ~ ẽĩ/ -un, -um, -ún, -úm3 /ũ/
The grapheme -en- is also pronounced as a nasal diphthong in a few compound words, such as bendito (bem + dito), homenzinho (homem + zinho), and Benfica.
Read more about this topic: Portuguese Alphabet, Vowels