Question

How do I know when to multiply divide subtract or add

Answer 1

Ummm if I understand the question there will be symbols.

Addition is +
Subtraction is -
Multiplication is * or x
Division is /

If you have an expression use PEMDAS like ChrisR described.

Answer 2

PEMDAS is very helpful to remember when thinking of order of operations. Parentheses, Exponents, Multiply, Divide, Add, then Subtract. Always in that order. 

If you are talking about something more specific you may want to give a bit more information.