If you here a ‘beep’ when trying to type something, there is a window somewhere asking for your attention. Look around, minimize all windows or press Alt-Tab (Mac: cmd-Tab) to see what it is.

It can depend on your version of Excel and your settings, whether formulas use the comma (,) to seperate arguments, or the semicolon (;). Excel gives you a suggestion once you start typing =IF( for example. You can spot the comma or semicolon directly.

Still nothing works? Shut down Excel and restart. Or restart your computer. (Really, in rare cases, this helps)

That's a good question.
By default, Microsoft-Office installs itself in the language of your computer, and that is usually fine. In the case of Excel, it means that all the functions--like SUM, AVERAGE, etc.--cannot be used like that. They have a localized name on your computer, like SOM, GEMIDDELDE-- in the case of Dutch versions of Excel. There are roughly two solutions:
1. Work with your version of Excel Google for the function names, i.e., how they are called in your language.
2. Install another version of Excel, or work on another computers.