Which one should I use: \begin{align} or \begin{aligned}?

Question

Which one should I use: \begin{align} or \begin{aligned}?

Answer 1

If you have a series of equations that you want to have mutual alignment, then you would use align. If the equations are part of a larger equation, then aligned is the right environment.

I often will use \begin{equation}\begin{aligned} ... \end{aligned}\end{equation} to give a single equation number to a group, for example if I'm showing steps.

Answer 2

align is used for entering to math-mode while aligned is used for multiple horizon alignment. For example

 \begin{align}
 \begin{aligned}
    2x+3 &= 7 & 2x+3-3 &= 7-3 \\
    2x &= 4 & \frac{2x}2 &= \frac42\\
    x &= 2
 \end{aligned}
 \end{align}

ps. the example is adapted from Math Mode document of Herbert Voß.

For grouping the multiple lines equation, you should use gather or multline.

Answer 3

You can use align inside subequation to get the numbering (1a) and (1b). Then you can refer the the equation system with (1) and to the subequations directly.

\begin{subequations}  
\begin{align}  
a &= b \\  
c &= d  
\end{align}  
\end{subequations}