For example:
(Independent sample) "In general, people's pulse is known. Investigate whether there is a difference between the pulse of people in an anxious state and the pulse in a general state. "
(Related samples) There is a one-to-one correspondence between the related samples of "investigating the income difference between husband and wife in the family"
I think generally speaking, comparing the differences between two people is an independent sample. Except for special cases such as husband and wife (taking family as their connection) and identical twin research (as a person), a person's reaction to two different things is a relevant sample.
The pre-test and post-test are related samples, because the same person will be tested twice, and the results of the pre-test and post-test will correspond to one person; The situation of the experimental group and the control group belong to independent samples, because people are divided into two categories, and each category receives an experimental treatment, such as some people A and some people B. No one can connect treatment A and treatment B because no one has received two treatments.