Pa is the thirteenth of the thirty consonants and opens the fourth line, the labials, made with the two lips. It writes a voiceless unaspirated p and, carrying the inherent vowel, is read pa. In modern Lhasa speech it takes high tone and has merged with pha, as the first two members of every line have merged.
Within the alphabet it is a root letter and nothing more. It takes superscripts and it takes the ya and ra subscripts, but it is not a prefix, a suffix, a superscript or a subscript itself.
Its subscript behaviour is worth flagging, because it is the clearest case in the script of writing and speech parting company. Pa with a subjoined ya is not pronounced as a p followed by a glide. In modern Lhasa speech the labials with a ya subscript are heard as palatals, so the stack that a learner would read as pya comes out closer to cha. The spelling is not wrong and the pronunciation is not careless; the two are related by a rule, and the rule has to be learnt as part of learning the letter.
Pa is also the letter of one of the most frequent grammatical elements in the language, which the following article takes up.
In modern Lhasa speech pa carries high tone, and it belongs with ka, ca and ta to the set of first-column letters that have merged with their aspirated partners. The merger is complete across all four of the regular lines, which is why the first two columns of the grid are now a distinction of spelling.