You can use your own keyboard to enter commonly used operators. The operators and their corresponding keys can be found in the table below. You can also use the keyboard arrows (← ↑ ↓ →) for traversing through your answer. In order to remove something, place your cursor behind it and press backspace.
You can also use a virtual keyboard, which does not only offer a copy of the usual keyboard, but has extra symbols which allow you to build more elaborate mathematical expressions, like a square root or a matrix (under tab "vector"). You can minimize or maximize the virtual keyboard by clicking on the tab at the top right, or change the available buttons per area by switching tabs. See the screenshot below.