gracy said:
Sorry I forgot to provide the page
View attachment 82968
It helps to make these molecules with model kits. With your model of the molecule (or with one in your head), put dots of paint on two of the hydrogen atoms. If you can do some non-bond-breaking kind of motion to the molecule to take one form into the other [i.e. they are identical, superimposable images (or mirror images) of each other] then the two forms tha tyou made are identical, and the two hydrogen atoms are chemically/physically identical.
With your specific question in mind, are all of the primary hydrogen atoms identical? If you put a dot of paint on one of the the left-most hydrogen atoms and a dot of paint on one of the bottom-most methyl group's hydrogen atoms, are these the same? They are not. You can do nothing that would convert the one kind of hydrogen atom into the other. One way to think about this is that the left-most methyl group is bonded to a secondary (methylene) carbon. While the bottom methyl group (and the right-most methyl group) are bonded to a carbon with a single hydrogen atom.
It may be hard to see, unless you can build a model, but the bottom methyl group and the right-most methyl group are chemically equvalent, but you may need to use a mirror to convert one form into the other.
Also note that the actual molecule shown in the picture is messed up. The second carbon should be a CH2 carbon, and the third carbon should have a single hydrogen. As drawn, the third carbon has five bonds, which is a no-no.