How to use reference to page number

For good or bad, I extensively use \autoref from the hyperref package. Mainly because it's simple.

Well there's a command named \autopageref{} which replaces \pageref{} and does exactly what I wanted. More info here.

How to write appendix to scrreprt

http://tex.stackexchange.com/questions/137150/appendix-title-with-scrreprt

Numbering equations like 1a and 1b

use align end subequations instead of eqnarray or equation

\begin{subequations}
    \begin{align}
        p(w) &\sim \mathcal{N}(0, Q)\\
        p(v) &\sim \mathcal{N}(0, R)
    \end{align}
\end{subequations}

 See here.

How to number several equations with one number

\begin{eqnarray} 
 \begin{array}{rl}
    &x = foo \\
    & y = bar
 \end{array}
\end{eqnarray}

 I sometimes get that I should not use eqnarray that extensive, but it works for me.
Another solutions with equation and aligned environments here.

write 1st, 2nd, ... nth correctly

Using the nth package as

\usepackage[super]{nth}

 ...

 \nth{<number>}

super is only needed if you want the "th" in superscript, this is the English preferred format, so I usually use as this

or alternatively

 k$^{\textnormal{th}}