Shortcut keys for inserting symbols and templates into the equation
To insert some symbols, you can press Ctrl+K, in the status bar of the Equation Editor dialog box you can see Symbol and than type next key:

I - Infinity
A - Arrow
D - Derivative (partial)
< - Less than or equal to
> -
Greater than or equal to
T - Times
E - Element of
Shift+E - Not an element of
C - Contained in
Shift+C - Not contained in
Inserting templates in an equation using keys
Templates can be inserted using a Ctrl+character combination or a Ctrl+T, character sequence.
Note: Some key combinations are not available with Windows NT.
Ctrl+9 or Ctrl+0 - Parentheses
Ctrl+[ or Ctrl+] - Brackets
Ctrl+{ or Ctrl+} - Braces
Ctrl+F - Fraction
Ctrl+/ - Slash fraction
Ctrl+H - Superscript (high)
Ctrl+L - Subscript (low)
Ctrl+J - Joint sub/superscript
Ctrl+I - Integral
Ctrl+T - Absolute value
Ctrl+R - Root
Ctrl+T, N - nth root
Ctrl+T, S - Summation
Ctrl+T, P - Product
Ctrl+T, M - Matrix template 3X3
Ctrl+T, U - Underscript (limit)
All of these symbols are characters from the Symbol font, so they can also be inserted by using their Alt key character codes.
Applying embellishments in an equation using keys
You can attach an embellishment to the selected character to the left of the insertion point with keyboard shortcuts.
Ctrl+Shift+Hyphen - Overbar
Ctrl+Shift+~ - Tilde
Ctrl+Alt+Hyphen - Arrow (vector)
Ctrl+Alt+' - Single prime
Ctrl+Shift+" - Double prime
Ctrl+Alt+Period - Single dot