We repeat. There is no need to remember all the details. A passive background knowledge that becomes active if a phenomenon shows up in a concrete context however can be useful. That sensitizes for the phenomena. In other words. It is well possible that someone doesn't here any difference between the nasal vowl in antes and the nasal vowel in falam. But if it had been pointed out before that there is a difference, it is more likely to pay attention to the difference and notice the difference.
In the IPA presentation a nasal vowel is marked with a til, a curving line upon the vowel.
IPA presentation
graphem
exemple
sound
[ɐ̃ ]
an
antes (before), transporte (transport)
,
[ɐ̃ ]
am
campo (land), samba (Samba)
,
[ɐ̃ ]
ã
irmã (sister)
Remark: The [ɐ̃ ] exists as well as a diphtong. In this case there is a semivowel, a vowel that is not pronounced entirely, attached to the vowel and this variation is very important, because the third person plural of the verbe in present and perfeito simples of the verbs finishing in -ar in plural as for instance falar (to speak) end with this diphtong which is written -am. (annotation: If we don't explain something in a certain context, it is not relevant in this context. About the perfeito simples we will discuss later on in detail, see 9.2. In this chapter for instance we don't care about the correct use of the verbal tenses. We are only interested in the pronunciation.) The IPA presentation of this diphtong is [ɐ̃w ] and the graphem, the way it is written, is presented with -ão or -am.
-am (as a dipthong) and -ão have both the same phonetic value [ɐ̃w ].
Eles falam = They speak.
Eles falarão = They will speak.
The graphem ã is always a monophtong. The graphem -am can be a monophtong or a diphtong. - ão is always a diphtong and has the same phonetic value as -am, if -am is pronounced as a diphtong. -am is only pronounced as a diphtong at the end of a syllable, what occures however very often, because the verbs whose infinitive ends in -ar end in -am in the third person plural.
IPA presentation
exemple
Monopthong
IPA presentation
exemple
Diphthong
sound
[ɐ̃ ]
amanhã =tomorrow
[ɐ̃w ]
eles falam = they speak
[ɐ̃ ]
maçã = apple
[ɐ̃w ]
eles falaram = they spoke
The graphem - am has two phonetic values. It represents the monophthong [ɐ̃ ] and, relevant most of all in the third person plural presente and perfeito simples of the verbs ending in -ar, the diphtong [ɐ̃w].
IPA presentation
exemple
IPA presentation
exemple
sound
[ɐ̃ ]
tambor = drum
[ɐ̃w ]
eles acabam = they finish
[ɐ̃ ]
maçã = appel
[ɐ̃w ]
eles falaram = they spoke
-am or -an are only pronounced as nasal vowel if they are not followed by a vowel. (People who have learned French at school knows that already.) If they are followed by another vowel they are just pronounced as an a followed by an m.
IPA presentation
exemple
oral
IPA presentation
exemple
nasal
sound
[a]
amor= love
[ɐ̃ ]
tambor = drum
This rule complies for all nasale vowels. The next table shows a list with all five nasale vowels, in both versions, with m and with n. In all these words am / an are not followed by another vowel. Those who paid attention in their French classes, a rare species of human beings, know that all combinations of en, em, an, am, in, im, un, um are not pronounced nasale if followed by another vowel (ambitieux (ambitious), nasale, but amer (bitter), not nasale).
IPA Zeichen
exemple
sound
[ɐ̃]
tampouco =neither
[ɐ̃]
ange = angel
[ẽ]
empregar = to employ
tr>
[ĩ]
ainda = still
[ĩ]
fim = end
[õ]
ombro = shoulder
[õ]
ontem = yesterday
[ũ]
jumbo = jumbo jet
[ũ]
junto = beside
For those who speak French or have learned it at school it is useful to see that apart of [õ] none of these nasale sounds correspond to the French nasal sounds. The graphems, the presentation in writing, exist in both languages, but the pronunciation is different. (Beside on / om which have the same phonetical value in French and Portuguese.) Furthermore Portuguese has one nasale vowel more. A nasale u doesn't exist in French.
graphem
exemple Fr.
exemple Po.
meaning
IPA Fr.
IPA Po.
pronunciation
em
employer
empregar
to employ
[ɑ̃]
[ẽ]
fr po
en
entre
entre
between
[ɑ̃]
[ẽ]
fr po
am
amplifier
amplificar
to extend
[ɑ̃]
[ɐ̃]
fr po
an
ange
anjo
angel
[ɑ̃]
[ɐ̃]
fr po
im
imbécil
imbecil
idiot
[ɛ̃]
[ĩ]
fr po
in
indigné
indignado
indignant
[ɛ̃]
[ĩ]
fr po
on
conte
conto
fairy tale
[õ]
[õ]
fr po
om
sombre
sombra
shadow
[õ]
[õ]
fr po
un
un
um
one
[œ̃]
[ũ]
fr po
um
-
cumprir
to accomplish
[œ̃]
-
- fr ton po
As already has been said the production of vowels are a little bit difficult to explain, because how they sound depends exclusively of the resonance body formed by the mouth. (Concerning the nose the question is only whether it is used as a resonance body or not, because we can't change it.) The best thing to do is to try. The Portuguese [ẽ] is produced further up than the French [ɑ̃]. That means: In the case of the French [ɑ̃] the back of the tongue is pressed against the palate at a deeper position in the mouth than in the case of the Portuguese [ɐ̃], wherin in both cases the back of the tongue doesn't touch the palate.
Summary:
1. The sound [ɑ̃] has three graphems: an, am and ã. The consonantes m, n are nasale consonantes. That a vowel in front of nasal consonantes becomes a nasal value is probably a celtic heritage.
2. The graphem an, am can be a monopthong or a diphtong. We will take a closer look on the diphtong later, see 3.3.2.
3. ã represents always a monophthong.
4. an am only represents a nasale vowel if not followed by a vowel.
5. The third person singular of the verbs ending in -ar, falar, presente and perfeito simples end in -am. In these case -am is to be pronounced as a nasal diphtong.
6. The third person plural future of the verbs end in ão. This has the same phontetic value as -am pronounced as a diphtong.
7. an or. am represents in French [ɑ̃], in Portuguese [ɐ̃].
Nobody knows really why and how, but human beings are able to imitate, with a little bit of practice, even sounds that doesn't exist in their mother tongue. The production of vowels is difficult to describe. But with a little bit of practice the reader will be able to produce them.