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.