Allan is right! In the world guia you read it as gi (as in give) because you have to know how to separate the syllable so gui/a . Other examples: guisado gui /sa/do, Guilherme, gui /lher /me . So when gui is all part of the same syllable you read it this way. In the word linguista you have lin/gu/is/ta so you have to read gu as in Guatemala. Other examples : arguir ar/gu/ir (arguing), exiguidade e/xi/gu/i/da/de, pinguim pin/gu/in it's not very easy but it gets instinctive once you ear the word one or two times. Hope it helps

G is pronounced as in 'game' before a, o and u → gato it is pronounced as the letter S in "mesure" before e and i → gelo For GUI it is difficult to explain because of a spelling reform in 2009 that made the writing of 'güi' change to 'gui' for but they are pronounced different, then in some words 'gui' is pronounced in two different ways. For example 'gui' in Linguista is pronounced kind of separated 'lingu-ista' while in Guia (guide) it is pronounced as in 'give'. Therefore there is no rule to pronounce GUI since trema (¨) does not exist in Portuguese language anymore, you have to learn the correct pronounciation of the words with GUI GUA is pronounced is pronounced in this 'separated' way that I said → "gu-arda" GUE is pronounced as in 'get'.

Hi Gill, I think that this question is difficult to answer you because we need to listen your pronounciation to correct you.