To write, letter by letter, I suggest you use something like this:
Code:
FONT* arial_font = "Arial#20b";
TEXT* mytxt = 
{
	layer = 1;
	pos_x = 100;
	pos_y = 100;
	string ("qwert yuiop asdfg hjkl zxcvb nm,. n qwert yuiop asdfg hjkl zxcvb nm,");
	font = arial_font;
}

void letter_by_letter(TEXT* _temp_field,int _string,int _delay)
{
	if(!_temp_field){return;}

	STRING* temp_string = "meow";

	TEXT* temp_text = txt_create(1,1);
	layer_sort(temp_text,_temp_field.layer+1);
	temp_text.font = _temp_field.font;
	temp_text.pos_x = _temp_field.pos_x;
	temp_text.pos_y = _temp_field.pos_y;
	temp_text.size_y = _temp_field.size_y;

	set(temp_text, SHOW);
	reset(_temp_field, SHOW);
	int i;
	for(i = 0;i<str_len((_temp_field.pstring)[_string]);i++)
	{
		str_cpy(temp_string,(_temp_field.pstring)[_string]);
		str_trunc(temp_string,str_len(temp_string)-i);
		str_cpy((temp_text.pstring)[0],temp_string);
		wait(_delay);
	}

	set(_temp_field, SHOW);
	ptr_remove(temp_text);
}

void main()
{
	level_load(0);

	wait(5);
	fps_max = 100;
	letter_by_letter(mytxt,0,5);
	wait(-2);
	letter_by_letter(mytxt,0,20);
}

It's not made by me, but I hope it helps anyway.

Greets


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