/i:/ is a long, front, close vowel. This sound should mean no problem to you as it is almost identical to the way we pronounce the letter ‘i’ in Spanish when we say ‘sí’.
The defining characteristic of a front vowel is that the tongue is positioned as far in front as possible in the mouth without creating a constriction that would be classified as a consonant. Front vowels are sometimes also called bright vowels because they are perceived as sounding brighter than the back vowels.
The defining characteristic of a close vowel is that the tongue is positioned as close as possible to the roof of the mouth without creating a constriction that would be classified as a consonant.
It is unrounded, which means that the lips are spread.
Examples of /i:/ are: