In the following chapters you will find a detailed description of the Portuguese phonetic system. This helps to be sensitized for the different phenomenons, but it is not of any use to learn these rules by heart and furthermore nobody will be able to memorize them. Throughout this book you HEAR these sounds SEVERAL THOUSANDS of times and if you know to what you have to pay attention you will learn how to pronouce them with without any effort. In some cases it may be helpful to memorize one word that contains this sound, then you will learn as well automatically the rule. In other words you can deduce the rule from this word. In part you will find orgies of rules, we will try to explain things as detailed as possible, but the only goal is to get sensitized for the different phenomenon. You should know the difference between an open e and a closed e, an open o and a closed o, the difference between voiced and unvoiced, the difference between a guttural r and a palatale r. It is useful to know what is a n / l mouillé and what is a monopthong and a diphthong. Some words like phonem and graphem are used in grammar books. It is useful to know what it is, but that is not a big deal. But to learn that at the beginning of a sentence the r is pronounced as a guttural r, that one r between two vowels is to be pronounced as palatale r, that rr is to be pronounced as a guttural r, that a r in front of a consonant is to be pronounced as a palatale r etc. etc. etc. is useless. It is enough to learn one word as an example. If you understood the rule, it is easy to deduce it based on this example.
Furthermore there are always in any language variations of a sound and in any language there is a discussion about which pronunciation is the best one. (Obviously everybody believes that his pronunciation is the best one. Very often, for instance in Italian and Portuguese, there is a debate whether the o / e should be pronounced as an open or a closed o / e. There is no need to have an opinion about that, actually it doesn't matter, but it can be useful to know what these people are talking about.)
As in any description of the phonetic system of a language we distinguish between vowels and consonants, because these are really two different things. In the case of a vowel the air flow is not interrupted by the tongue, the teeth, soft palate. The different sounds are produced by varying the resonance body formed exclusively by the mouth, in case of oral vowels, or with the mouth and the nose, in case of a nasale vowel. Vowels are always voiced, in other words the vocal chords vibrate. If there is no friction, if the air flow just flows out without any hindrance, there would be no sound at all without the vocal chords. That the same thing as with a guitar. If the chords doesn't vibrate, there is no sound at all, although you can produce the needed frictions in another way, for instance by using the guitar as a drum.
Vowels are more difficult to describe than consonants, because how they sound depends exclusively on the resonance body and the resonance body is difficult to describe. Furthermore the transition from one vowel to another vowel is fluent. The pronunciation of the diphthong in a simple word such as day is different with a british, american or australian accent, although the differences are difficult to describe, although they are easy to hear.
Consonants are in general easier to describe because they are produced in a specific way and the difference between one consonant and another is very clear. There are, in most cases, the r is a more complex example, no fluent transitions. There is a clear distinction between t and d, between a voiced sh and an unvoiced sh, between m and n etc.. The bigger problem are the vowels, not the consonants.
We present for the relevant sounds, those who can cause problems, the IPA (International Phonetic Associacion) signs. We will see, that this is sometimes helpful. We focus on the sounds that doesn't exist in English. There is no need to describe the sounds that are pronounced in English the same way as in Portuguese, because they don't present any problem, although the graphem, the way they are spelled, is sometimes different. In general human beings are able as well to imitate sounds they don't have in their mother tongue. "Unconsciously" people have a good understanding how they can produce different sounds, even if they don't really know how it works, but a little practice is needed.