What are the 18 major parts of typewriter?

Assuming you mean a manual typewriter, here's what I can think of:

1) Keys (alphanumeric, symbol and function - could count as several "parts")

2) Spacebar

3) Carriage return / line space lever

4) Action bars

5) Typebars (known collectively as the typebasket)

6) Typeslugs (top of the typebars, with text dies on them)

7) Escapement mechanism

8) Carriage return spring

9) Draw-band

10) Carriage support (a separate item on some designs like the Olympia SG1)

11) Carriage

12) Frame

13) Shell

14) Feet

15) Carriage shift mechanism, or typebasket shift mechanism (depends on design)

16) Platen

17) Platen rollers

18) Paper table

19) Paper guide & guide plate

20) Paper bail & bail rollers

21) Platen knob(s) - usually two, sometimes just one on compact portables

22) Line space selector / indicator

23) Line space ratchet release

24) Carriage release

25) Margin indicators

26) Right margin bell

27) Margin release

28) Tab set key (if equipped)

29) Tab release (clears all tabs) and/or tab clear (one tab only)

30) Ribbon trays

31) Ribbon vibrator

32) Aligning scale

33) Card holder

...And I could go on. Pick the ones that seem most important to you. :-)