"I open the letter."
Translation:Saya membuka suratnya.
I am not a native speaker but in the absence of any other answers:
The '-nya' suffix has the meaning of the/his/hers/theirs, however it is usually only used when it is very obvious from the context which letter you are talking about. It is less common to hear someone use '-nya' if no letter had already been referred to in the conversation. Of course there are exceptions, but this is common when '-nya' is put at the end of a physical object such as a letter or a book.
In this case, if the letter is being mentioned for the first time, it would be more common in my experience to hear 'surat itu' or 'surat dia/mereka' and then later in the conversation revert to 'suratnya'.
Great info about 'nya'. This makes a lot more sense now, since books and letters often do belong to someone. Thanks for sharing your knowledge!