How do you know?
If you can divide `360^@` exactly by an inside angle of a regular shape and get a whole number it can form a tessellation pattern.
(6 is a whole number)
6 therefore this triangle can form a tessellation.
(4 is a whole number)
A square can form a tessellation.
(3.33 is NOT a whole number)
A pentagon can NOT form a tessellation.
A hexagon can be tessellated.