Hi, this is a dumb question but can someone please explain to me when you're meant to use this confidence interval:

$\displaystyle \mu +- t_{\alpha/2, n -1} \sqrt{\frac{\sigma^2}{n}} $

and this one:

$\displaystyle \mu +- t_{ 1 - \alpha/2, 2n -2} \sqrt{\frac{\sigma^2}{n}} $

coz I keep using the wrong one at the wrong time so i keep getting my answers wrong