name an isomer of pentane (that is not cyclic)and write down its structure

Answer 1

C-C-C-C-C with all bonds filled with H is n-pentane.
C
|
C-C
|
C
|
C
with all the bonds filled with H is 2-pentane or isopentane. (There is a third one; i.e., neopentane, in which the lowest C on the second structure is attached to the #2 carbon--the same one to which the other CH3 is attached.)

Answer 2

I didn't name the isopentane (which is correct) correctly. It should be called 2-methyl butane.