The two-dotted i: the "i" disappears

Question

I need to type the word "naïve" in plain text in Latex. Things that I've tried: na\"{\i}ve, na{\"{\i}}ve, na\"\i ve, and other combinations of brackets: i dissappears, and this is what I get:

I tried to switch to math mode and then it works, but I cannot make the \imath to look non-italic, this is what I get from $\mathrm{na\ddot{\imath}ve}$ :

EDIT: Added minimum working example.

\documentclass[a4paper,11pt]{book}
% bibentry workaround
\makeatletter
\let\saved@bibitem\@bibitem
\makeatother
\usepackage{bibentry}
\nobibliography*

\usepackage[english]{babel}

\usepackage{calc}           % berekeningen voor paginagrootte
\usepackage{afterpage}      % om een \clearpage na einde van pagina te zetten
\usepackage{titlesec}
\usepackage{fancyhdr}

\usepackage[pdftex=true,hyperindex=true,colorlinks=false,hidelinks=true]{hyperref}
\hypersetup{%
    pdfauthor={...},
    pdftitle={...},
    pdfsubject={...}
}

\usepackage[english,tight]{minitoc}
\setcounter{minitocdepth}{2}

\usepackage[pdftex]{graphicx}
\DeclareGraphicsExtensions{.jpg, .jpeg, .png}
\graphicspath{{../images/}{images/}}

\usepackage{wrapfig}
\usepackage{subfig}
\usepackage[labelfont=bf,font=small]{caption}
\captionsetup[subfloat]{margin=4pt}

\usepackage{color}
\usepackage[table]{xcolor}

\usepackage{listings}
\lstset{%
    flexiblecolumns=true,
    basicstyle=\ttfamily\footnotesize,
    basewidth=0.4em,
    aboveskip=5mm,
    belowskip=2mm,
    frame=tb,
    frameround=fttt,
    framexleftmargin=2mm,
    framexrightmargin=2mm,
    framextopmargin=1mm,
    framexbottommargin=1mm,
    showspaces=false,
    showstringspaces=false,
    mathescape,
    captionpos=b
}

\lstdefinelanguage{XML}{
    morestring=[b]",
    morestring=[s]{>}{<},
    morecomment=[s]{<?}{?>},
    stringstyle=\color{black},
    identifierstyle=\color{blue},
    keywordstyle=\color{magenta},
    morekeywords={role,content,action,level}
}

\usepackage{url}
\usepackage{array}
\usepackage[figuresright]{rotating}
\usepackage{amssymb,amsmath}

\usepackage[square,comma]{natbib}
\usepackage{appendix}
\usepackage{makeidx}

\usepackage{pdfpages}

%
% new/renew commands
%
\renewcommand{\mtcfont}{\small\rm}
\renewcommand{\mtcSfont}{\small\bf}
\renewcommand{\cite}[1]{\citep{#1}}
\renewcommand{\i}[1]{#1\index{#1}}

%
% make appropriate UHasselt style
%
\titleformat{\chapter}[display]
{\bfseries\Large} 
{\filright{\chaptertitlename} \Huge\thechapter}
{4ex}
{\titlerule\vspace{2ex}\filcenter}
[\vspace{2ex}\titlerule]

\setlength\oddsidemargin{4cm - 1in}
\setlength\evensidemargin{4cm - 1in}
\setlength\textwidth{13cm}

\setlength\topmargin{48mm - 1in}
\setlength\headheight{26pt}
\setlength\headsep{10pt}
\setlength\textheight{20cm - \headheight - \headsep}

\pagestyle{fancyplain}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\renewcommand{\sectionmark}[1]{\markright{\thesection\ #1}{}}
\lhead[\fancyplain{}{\bfseries\thepage}]{\fancyplain{}{\bfseries\rightmark}}
\rhead[\fancyplain{}{\bfseries\leftmark}]{\fancyplain{}{\bfseries\thepage}}
\cfoot{}

%
% custom stuff
%
\vfuzz=2pt % don't report over-full v-boxes if over-edge is small
\hfuzz=2pt % don't report over-full h-boxes if over-edge is small

%\usepackage{tocloft}
%\@addtoreset{section}{part}
%\renewcommand\thepart{\arabic{part}}

%
% prepare things...
%
\def\C++{\leavevmode{\hbox{C\hskip -0.1ex\raise 0.5ex\hbox{\tiny ++}}}} % nice C++ logo
\def\Csharp{\leavevmode{\hbox{C\hskip -0.1ex\raise 0.5ex\hbox{\tiny \#}}}} % nice C# logo

\makeindex
\dominitoc

\begin{document}
    na\"{\i}ve
\end{document}

What could possibly be wrong?

Answer 1

Update for question with MWE (version 3)

The OT1 encoding composes the ï with the accent (\") and the dotless i (\i), because there is no slot available in the 7-bit font encoding. The T1 encoding has slot 239 for this glyph (from t1enc.def):

\DeclareTextComposite{\"}{T1}{i}{239}
\DeclareTextComposite{\"}{T1}{\i}{239}

Therefore the form \"i works without an explicit or implicit \i, thus that \i can even be redefined:

\documentclass{article}
\usepackage[T1]{fontenc}
\renewcommand{\i}[1]{#1\index{#1}}
\begin{document}
na\"ive
\end{document}

 

Older version

^{Wild guess, based on version 2 of the question without minimal working example (MWE).}

The accent command \" is a text command that causes an error in math mode (your question is tagged with math-mode). Switch to text mode to typeset the word, e.g.:

\documentclass{article}
\usepackage{amstext}% or amsmath, provides \text
\begin{document}
\[ \mbox{na\"ive} = \text{na\"ive} \]
\end{document}

Answer 2

The definition of \i as the lazy LaTeX-User form for \index seems to be the cause of the issue – Christian Hupfer.

Commenting the line \renewcommand{\i}[1]{#1\index{#1}} fixed the problem.

Thanks!!