To access special characters on an iPhone keyboard, you can press and hold the corresponding letter or symbol. For example, if you need to type an accented letter, such as "é", you can hold down the "e" key and select the accented version from the pop-up menu. Similarly, you can hold down the currency symbol to access alternate currency options, or hold down the number key to access mathematical symbols. If you frequently use certain special characters, you can also add them to your "Favorites" tab for easy access. To do this, go to Settings > General > Keyboard > Keyboards > Add New Keyboard, then select the language and keyboard type that includes the characters you need. Once the new keyboard is added, you can switch between it and your primary keyboard by tapping the globe icon on the keyboard.