PART 1 (See part 2 on my comments section) If I understood you correctly, what you actually want to know is when to pronounce them one way or another. Is this correct? If so, then the answer is, it depends. We need to classify these syllables in 2 categories. - DI and TI - DE and TE And give two positional conditions: - Stressed (syllable) - Unstressed (syllable) The way you pronounce them will depend on these 4 factors: - DI in a stressed syllable - DI in an unstressed syllable - TI in a stressed syllable - TI in an unstressed syllable - DE in a stressed syllable - DE in an unstressed syllable - TI in a stressed syllable - TI in an unstressed syllable DI This will always be pronounced like the English letter "j" in "jeep" (more or less like "djee") when they are in a stressed syllable. For example, in "dia" (day), "di" is stressed, so that'll be "jee". When they are in an unstressed syllable, they will still have a /j/ sound. However, they will be pronounced more like "zh" (rather than "jee"). As in, e.g., the word "homage". In other words, you will place your lips in the "i" position without actually pronouncing the vowel; then say "zh...". TI Pretty much all I said for "DI" should be applied here, except you will use the unvoiced counterpart: "ch" (as in "check") rather than /j/ or /zh/. -- I’m sorry. After writing my explanation I realized it was too long for the website. I tried shortening it, but I thought important pieces were missing for your understanding, so I’ve cut my answer down in half, and posted its first part here. To see the second part, please click comments below to see the remainder of it.

That's quite right! We actually use "Dg" and "Tch" sounds: "Bom dgia", "dgiferentchi"... This is really common, almost in all of the country except for some cities in the country side of Sao Paulo state (and even in those cities, almost only old native people don't use these sounds)

People of some regions in Brazil pronounce the word "DIA": "DEEya", while those from other regions say "GEEya". It doesn’t mean one way is more correct than the other,since in Brazil pronunciation tends to be more a regional distinction than a class one. So, don't be confused about pronunciation :)

that's right if you go to brazil but if you go to Por tugal you will say Bom dia exactly as you write it.

In differents languages we use differents muscles of the mouth. In Brazil we have some difficulties to pronunciate "Think" "Thank you" "Really" because we use different muscles of the mouth. Maybe you could to try this video: https://www.youtube.com/watch?v=HvHCz8I5OSU