When to use T test vs Z test?

Hello! 

Thank you for your question. Both of these tests are applicable when dealing with statistics.
 A Z-test is a statistical test used to determine whether two population means are different when the variances are known and the sample size is large.  A T-test is a statistical test used to determine if there is a significant difference between the means of two groups, which may be related in certain features.
A Z-test is usually done when the sample size is larger, often 30 or more,  whereas a T-test is done with smaller sample sizes, 30 or less. 
Hope this helps!