if vs. whether vs. when

Use if to express a condition, use whether to express uncertainty, and use when for situations that require preparation or to denote the passage of time.

In informal writing for the web, it's OK to use if to express uncertainty. Don't use whether or not to express uncertainty.

Examples
If you don't know whether a network key is needed, contact your network administrator.
Use your BitLocker recovery key to sign in if you're locked out of your computer after too many failed password attempts.
When Setup is complete, restart your computer.
To find out whether TrueType fonts are available ….
(Informal) To find out if TrueType fonts are available ….
Save a webpage to view it later, even if you're not connected to the internet.