Both structures have the same chemical formula (C₆H₁₄), so they might be isomers.  They have the same sequence of atom connections (a five-carbon chain with a methyl group attached to the second carbon), so they are not constitutional isomers.  They can be interchanged by rotation around a single bond followed by rotation of the entire structure, so they are not stereoisomers.  (Verify this by examining molecular models.)  These molecules are, in fact, identical and not isomers at all.

Both structures have the same chemical formula (C₆H₁₄), so they are isomers. They do not have the same sequence of atom connections, so they are constitutional isomers.

Both structures have the same chemical formula (C₈H₁₆), so they are isomers.  They have the same sequence of atom connections (a cyclohexane ring with methyl groups on adjacent carbon atoms), so they are not constitutional isomers.  The structures cannot be interchanged by rotation around single bonds (verify this with molecular models) so they are stereoisomers.  They are not mirror images (again, verify with molecular models) so they are diastereomers.

A quick inspection shows these molecules to be very similar.  Rotation around one bond makes them even more similar:

The molecules have the same chemical formula (C₁₃H₁₀N₂O₄), so they are isomers.  The sequence of atom attachments is identical, so the molecules are not constitutional isomers.  They are not identical, so they are stereoisomers.  The molecules are mirror images, so they are a pair of enantiomers.

These molecules have the same formula (C₁₉H₂₈O₂), so they are isomers.  The sequence of atom attachment is identical, so they are not constitutional isomers.  The molecules are not mirror images nor are they identical, so they are diastereomers.

These molecules have the same chemical formula (C₇H₁₄O₂) so they are isomers.  They have the same sequence of atom attachment (a cyclohexane ring with OH and OCH₃ on adjacent carbons), so they are not constitutional isomers.  Careful inspection of molecular models reveals that these structures are nonsuperposable mirror images, so they are enantiomers.

• Classification of Isomers Tutorial and Exercises
• Tutorial Topic List
• Stereochemistry Practice Problems