Knowing the meaning of a word is just one part of truly mastering a word. Another important part of vocabulary mastery is knowing the different forms of the word. What does the word look like when it is being used as a noun? What about as a verb?
In most cases, the word will change slightly depending on the part of speech being used. For example, the word understand is a verb; however, when used as a noun, it becomes understanding. Then, as an adjective it is understandable. The root of the word stays the same while the ending of the word changes according to the target part of speech.
Unfortunately, this is not always the case. Some words, like level, do not change with every part of speech. As both a noun and a verb, level stays the same. Also, the endings that are added to words when they do change, are not always the same for the same part of speech. For example, you add the -ing ending to the verb understand to make it a noun while you add the -ance ending to the verb rely to make it a noun, reliance.
As a result, there are no set rules for knowing when a word changes or how to change it. The different forms of a word just need to be memorized. Luckily, however, there are some common endings, suffixes, for each part of speech. These suffixes can be applied to a word to make it the target part of speech, and, through trial and error, the learner can find the best form for their context.