How to print trailing spaces?

edit

Since the show command uses LaTeX to typeset, you can use the ~ LaTeX command. Try this:

reset()
var('x t')
sp = LatexExpr('~')
Psi = function('Psi')(x, t)
V = function('V')(x)
show('Plug', sp, Psi, ', ', diff(Psi, t), ', ', diff(Psi, x, x), sp, 'into', sp, V)

The result is $\newcommand{\Bold}[1]{\mathbf{#1}}\verb|Plug| ~ \Psi\left(x, t\right) \verb|,| \frac{\partial}{\partial t}\Psi\left(x, t\right) \verb|,| \frac{\partial^{2}}{(\partial x)^{2}}\Psi\left(x, t\right) ~ \verb|into| ~ V\left(x\right)$

You can gain more control on spacing and fonts if you also use the html function and the format method, since then you can employ the resources of the HTML language. Try this:

reset()
var('x t')
Psi = function('Psi')(x, t)
V = function('V')(x)
format_string = '<strong>Instructions</strong><br>Plug ${}$, ${}$ and ${}$ into ${}$'
terms = [latex(i) for i in [Psi, diff(Psi, t), diff(Psi, x, x), V]]
show(html(format_string.format(*terms)))

The output is:

Instructions
Plug $\Psi\left(x, t\right)$, $\frac{\partial}{\partial t}\Psi\left(x, t\right)$ and $\frac{\partial^{2}}{(\partial x)^{2}}\Psi\left(x, t\right)$ into $V\left(x\right)$

You can use tabulator character "\t":

show('Plug\t', Psi, ', ', diff(Psi, t), ', ', diff(Psi, x, x), ' into\t', V)

$\newcommand{\Bold}[1]{\mathbf{#1}}\verb|Plug | \Psi\left(x, t\right) \verb|,| \frac{\partial}{\partial t}\Psi\left(x, t\right) \verb|,| \frac{\partial^{2}}{(\partial x)^{2}}\Psi\left(x, t\right) \phantom{\verb!x!}\verb|into | V\left(x\right)$

Or if you want a bigger gap add as many spaces before "\t" as you want:

show('Plug     \t', Psi, ', ', diff(Psi, t), ', ', diff(Psi, x, x), ' into     \t', V)

$\newcommand{\Bold}[1]{\mathbf{#1}}\verb|Plug|\phantom{\verb!xxxxx!}\verb| | \Psi\left(x, t\right) \verb|,| \frac{\partial}{\partial t}\Psi\left(x, t\right) \verb|,| \frac{\partial^{2}}{(\partial x)^{2}}\Psi\left(x, t\right) \phantom{\verb!x!}\verb|into|\phantom{\verb!xxxxx!}\verb| | V\left(x\right)$