The short sound is 'i' and sounds like 'i' of ink, think, sing etc.
It is said for 1 unit of time. The longer vowel, or diirgha is said for 2 units of sounds (represented in ITRANS as ee, ii, or I). The longer vowel for 3 units is not used in classical spoken Sanskrit, and is only used in Vedic chants, and is denoted by i3, the 3 denoting 3 units of time. malayAlam for one, has longer vowels as well, so I am told.
Writing i and ii is easy (see below). It is like an 'S' hanging from a stalk. But, the bottom of the S has one more curved 'knot', without which it would be the consonant 'D' sound.
The word 'i', or the verb root 'i' means - to go, to go near. the verb forms go like
एति इतः यन्ति (eti, itaH, yanti)
एषि इथः इथ (eShi ithaH itha)
एमि इवः इमः (emi ivaH imaH)
e.g. निर्बुद्धिः क्षयं एति nirbuddhiH (mindless) kShayam (to ruin) eti (goes)
