"She writes a letter."
Translation:Ia menulis surat.
If you confuse between the two, just use the base verb in your daily conversation.
Base verb of "menulis" is "tulis" (to write)
Base verb of "membaca" is "baca" (to read).
Saya tulis buku / Saya nulis buku- I write a book
Saya baca buku - I write a book.
By using this base verb, you can distinguish the words better. Plus, it's now more informal, so it is more natural to use among friends.