The user interface for the Wolfram System provides many options for formatting input and output.

The sum of from to is :

While correct, this does not look like traditional mathematics. Select the cell group, then choose the menu item Cell ▶ Convert To ▶ TraditionalForm:

Alternatively, if you only want to format the answer, apply TraditionalForm:

InputForm and StandardForm work in reverse.

Use InputForm to find out how to type an expression into the Wolfram Language:

StandardForm is more compact than InputForm and is unambiguous to the Wolfram Language:

TraditionalForm is sometimes ambiguous; in this case, the Wolfram Language treats f first as a number and then as a function: