Open any page of written Tibetan and the syllable pa, or its variant ba, will be on it several times. It is the nominalising particle, and its job is to turn a verb or a phrase into a thing: an act into an actor, a process into a fact, a clause into something a sentence can be built around.
It is also part of a great many ordinary words, and it attaches to the ends of names and titles, so a reader meets it as a suffix on vocabulary as much as a piece of live grammar. Whether pa or ba appears is determined by the letter that ends the preceding syllable, which is one of a family of such rules in Tibetan: the case particles, the connective particles and the ending particles all take a form dictated by the final of the syllable before them.
That is a distinctive feature of the language and it has a practical effect on reading. The particles are short, they are frequent, and their shape is predictable from context, so an experienced reader stops decoding them and begins to see them as punctuation, marking where a clause turns. A learner who has not yet reached that point finds the same syllables the hardest part of the page, because they are the parts that are not vocabulary and cannot be looked up profitably.
The letter pa, in other words, is met far more often as grammar than as the head of a word.