There are full vowel signs and attached vowel signs for (almost) every vowel – in contrast to consonants who have only a full consonants sign.
In this part I focus on the first six vowels.
अ अा इ र्इ उ ऊ – a aa i ii u uu
The full vowel sign is used if the vowel is NOT attached to a consonant. Which is the case if the vowel stands either in the beginning of a word or if it follows another follow (vowels can only be attached to consonants!).
Only for the letters अ अा there is actually a difference in pronunciation in Nepali. The I and the U letters are pronounced differently (short vowel vs. long vowel) in Sanskrit but have lost the differentiation of pronunciation in Nepali. Of course the same holds true for the attached vowel signs.
The attached vowel signs are positioned before, above (wait for the next parts), behind or below the consonant, but are ALWAYS pronounced AFTER the consonant!
The attached vowel sign for a / अ is inherent already in the consonant sign. This means that any written consonant is actually a sign for a syllable already – e.g. म stands for ma