Nga is the fourth of the thirty consonants and the nasal that closes the velar line. It writes the sound English spells ng in sing, and unlike English it is free to begin a word: nga is the ordinary Tibetan for I, and a speaker uses it constantly.
Its position follows the inherited Indian arrangement exactly. Each of the first lines runs through its stops and then closes on the nasal formed at the same point in the mouth, so a reciter passes from ka to kha to ga and lands on nga, the velar nasal, before moving forward to the palatals. The pattern is the reason the recitation is memorable: the mouth works from the back to the front in four steps and repeats the same manner sequence at each stop.
Nga is one of the ten suffixes and closes a very large number of Tibetan syllables. It is not among the five prefixes, so unlike ga and da it does its extra work only at the end of a syllable and never at the beginning. As an initial it carries low tone in modern Lhasa speech, in company with the other nasals and with the letters that were voiced in the classical language.
For an English reader the letter is a small correction to an assumption. The sound is not a cluster of n and g, and it is not confined to the middle and end of words; it is a single consonant that Tibetan treats as fully as usable at the front of a word as any other.