Functions in Mathematica’s native format looks different for two reasons. The first character is capitalized no matter what. Even the Euler’s constant e and the imaginary i are capitalized. No exceptions.
And the function parameters are grouped by brackets, not parenthesis. This eliminates any ambiguity whether f(x) is f times x or f of x. In Mathematica f(x) is always f times x and f[x] is f of x. (For arrays, double bracket is used. The first element of the array f would be expressed as f[[1]].)
Thus, sin(x) should be entered as Sin[x]. 
There is “Basic Commands” pallet to guide the users. First, open “Basic Math Assistant” pallet.
then open “Basic Commands” section.
Frequently used mathematical functions as well as constant such as π can be found from the “Basic Commands” menu. You can find more functions/constants by dropping down the “More” menu in each section.
