So when doing some of the qbank questions i came across something i dont get: when we find the test statistic whether using t_test or ztest, how do you know whether to use the std dev on the denominator or std_dev/sqrt(n) ?? Thanks!

The std_dev/sqrt(n) corresponds with the distribution of the sample mean and not the sample. So if your test statistic is about the sample mean distribution you should use std_dev/sqrt (n). When the test statistic is about the sample distribution you should use std_dev. good luck with it. Pieuwie