For a short written answer, use a Long Answer block with a small mark allocation. There is no separate short-answer block in the New editor.
Setting one up
Click into the page where you want it, type /, and pick from the menu. Typing after the slash filters the list.
Choose Long Answer, write the question and set the marks to 1 or 2.
Write the mark scheme for a one-word answer
This is where short answers go wrong. Give the acceptable alternatives, or the marker rejects a right answer phrased differently: "Accept chloroplast, chloroplasts, or the chloroplast." For a calculation, give the range you will accept rather than one exact figure.

