Without both hands, typing on the full-sized keyboard can be difficult. Instead, you can shrink the keyboard to the right or left of the screen.
Open an app that accepts text, such as Notes or Mail. Tap in any text-based field or other area to display the keyboard. Press down on the globe icon at the bottom left. You’ll see three icons for the keyboard. Tap the one on the left to move the keyboard to the left. Tap the one on the right to move the keyboard to the right.
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You should now find it more comfortable to tap on the virtual keys with just the thumb of one hand. To return to the full-sized keyboard, press the left or right arrow.