Q:

Writing a text side by side an equation in \align environment

I have a very long equation in \align* environment so that I split it into two lines as shown in the screenshot blew 

I want to write some text side to side the above part of the equation using &&\text{} ,but Latex throw it away in the margins and I do not know why this happens. Here is my code regarding this part :
\begin{align*}
X_{p}\psi &=  
X_p\bigl( \psi(p) \bigr) 
 + \sum_i \left(\left.\frac{\partial \psi}{\partial x^{i}}\right|_{p} \right) X_{p}\left(x^{i}-a^{i}\right)           &&\text{(plaplapla)} \\
&\phantom{{}= }+ \sum_i \sum_j X_{p}\left( \left(\zeta_{ij}\circ\phi \right)\left(x^{i}-a^{i}\right)\left(x^{j}-a^{j} \right) \right) .

The first & is using for aligning equations later, any my preamble just has the necessary packages for mathematics. 
I appreciate any help. Thanks in advance.

A:

There are several possible solutions to this problem, depending both on the real equations and the real hints, and also on the page layout. It's easier if you load the geometry package, because the default horizontal margins with geometry are much more sensible. Second, you can use for the second line (without hint) the \mathrlap command from mathtools (which loads amsmath, so don't load the latter). alignat would let you control the horizontal spacing between equation and hints.
Here is an example:
\documentclass[a4paper,]{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage[showframe]{geometry}%

\begin{document}

\begin{align*}
  X_{p}ψ&=
  X_p\bigl( ψ(p) \bigr)
  + ∑_i \left(\left.\frac{∂ ψ}{∂ xⁱ}\right|_{p} \right) X_{p}\left(xⁱ-aⁱ\right) &   & \text{some more or less long hint} \\
  &\phantom{{}= }+ ∑_i ∑_j \mathrlap{X_{p}\left( \left(\zeta_{ij} ∘ \phi \right)\left(xⁱ-aⁱ\right)\left(x^{j}-a^{j} \right) \right)+A+B.}
\end{align*}

\end{document} 

A:

Assuming the explanatory text string isn't all that long, it should be enough to separate it from the other material in the first row with a \quad or \qquad directive.
I would also like to recommend strongly that you get rid of the habit of using \left and \right with all math-mode parentheses. See, e.g., Is it ever bad to use \left and \right? for more information on this subject.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\hrule % Just to illustrate the width of the text block -- feel free to omit!
\begin{align*}
X_{p}\psi &= X_p\bigl( \psi(p) \bigr) 
  +\sum_i \biggl(\frac{\partial\psi}{\partial x^{i}}\biggr|_{p}\,\biggr) X_{p}(x^{i}-a^{i}) 
  \qquad\text{some explanatory thoughts}\\
&\quad+ \sum_i \sum_j X_{p}\bigl( (\zeta_{ij}\circ\phi)(x^{i}-a^{i})(x^{j}-a^{j}) \bigr) .
\end{align*}
\end{document}

