"Dia menulis surat" can be often interpreted as "S/he writes letters". It would be more accurate if it is written as "Dia menulis sebuah surat."
What does "sebuah" mean?
Sebuah is "a"
I would say "sepucuk surat" instead of "sebuah"
Often the word "dia" shows up as "ia". Is this an informal pronoun or is there a misspelling in the question?
Dia = Ia.
'Dia' could only be used for he/she, While 'Ia' could also be used for inanimate objects, and also 'Ia' sounds more poetic.
Kucing nya bisa nulis surat