'hEEft' and 'EEn' - both have a 'ee' vowel combination, which doesn't appear in the list of 16 vowel combinations, although there is a 'edu' combination.
How does this 'ee' combination fit in with the list of 16, and is the 'ee' combination really a combination, or just two 'e's together?
The vowel is doubled in closed syllables to keep the long vowel sound. For example: In "de vader" the 'a' sound is long, because the 'd' is part of the second syllable so the 'a' is in an open syllable (va-der). In "het bed" the 'e' sound is short, because the syllable is closed by the 'd'. So, "en" ("and") has a short vowel sound and "een" ("a" or "an") has a long vowel sound. To keep the short vowel sound in the plural, "het bed" doubles the last consonant "de bedden" (bed-den). https://www.duolingo.com/comment/3733043
So it did not appear in the vowel combinations, since it is not really a vowel combination, but the difference between a long or short vowel sound.
Here are two sites that are very helpful for Dutch pronunciation: