I briefly alluded to broken plurals at the beginning of this chapter. These are plurals which arise from internal and sometimes external changes to the singular noun. For example, the plural of كتاب is كتب The word كتب is a broken plural.
Since you have had Arabic before, you know that broken plurals exist in many patterns. My advice is not to try to memorize the patterns themselves. Instead, learn the plurals of new words as you come to them. In time you will become familiar with the patterns themselves and may even be able to produce the plurals for new vocabulary items on the basis of intuition and experience. Lesson 13 of EMSA lists the plurals of some very common words and incorporates many of the major patterns. You might want to take a look at that lesson. Keep in mind that adjectives as well as nouns can have broken plurals. Also be aware that some words may have more than one broken plural, and that sometimes these different plurals will have different meanings.
It is important, very important, that you learn the plurals of new words as you come to them. (The dictionary by Hans Wehr usually gives the plural under the entry for the singular.) This will help when you see new vocabulary items in authentic Arabic texts which are already plurals. You need to be able to recognize that a new word may already be plural so that you can discern the root, make a good guess at what the singular is, and therefore be able to find the word in the dictionary.
Again, remember that adjectives as well as nouns have broken plurals.