A method for adding words to an online dictionary used for spellchecking is described. A spellchecker module compares words of an electronic document with words in the online dictionary and identifies a word in the electronic document that is missing from the dictionary. After a user indicates a desire to add the missing word to the dictionary, the spellchecker module determines at least one related-word form of the missing word. The related-word forms depend upon the part of speech of the missing word. The spellchecker can prompt the user to identify the part of speech and then to verify each determined related-word form. The spellchecker concurrently adds the missing word and at least one related-word form of the missing word to the online dictionary in a single `add-to-dictionary` operation.