Me- marks an active verb where the subject is doing something to something. It is the single most common affix in the language, and it is the reason Indonesian words look unfamiliar.
It is written meN- in grammar books, where the capital N stands for “a nasal sound that changes”.
The problem in one line
me- + tulis is not metulis. It is menulis, and the t has vanished.
The four faces
mem- before b, p, f
men- before d, t, c, j
meng- before vowels, g, h, k
meny- before s
membaca from baca ‘read’
mendengar from dengar ‘hear’
mengambil from ambil ‘take’
menyapu from sapu ‘sweep’
Why letters disappear
When the root starts with p, t, k or s, that letter is swallowed by the nasal.
potong ‘to cut’ → memotong. The p is gone.
tulis ‘to write’ → menulis. The t is gone.
kirim ‘to send’ → mengirim. The k is gone.
sapu ‘to sweep’ → menyapu. The s is gone.
Their voiced partners survive untouched, which is the pattern underneath:
baca ‘to read’ → membaca. The b stays.
dengar ‘to hear’ → mendengar. The d stays.
ganti ‘to change’ → mengganti. The g stays.
jual ‘to sell’ → menjual. The j stays.
So: p, t, k and s disappear. Everything else stays.
This is why stripping a me- word is not always obvious. Menulis looks like it should come from nulis, and nulis is in fact what people say in casual speech, which does not help.
Why bother, if speech drops it
Two reasons. Written Indonesian keeps the prefix everywhere, and signs, forms, news and official speech are full of it. And the passive di- form, which you will meet next, keeps the root visible, so knowing which root a me- verb came from lets you recognise its passive twin.
What you’ll actually hear
What the book says: Saya sedang menulis surat.
What you’ll hear: Aku lagi nulis surat.
Casual Indonesian drops the me- and keeps the changed root. menulis becomes nulis, membaca becomes baca, mengambil becomes ngambil.
The full rules, including menge- for one-syllable roots, are the first chapter of B1.