Introduction Many students have difficulty in terms of inflectional affixes and derivational one. Before discussing the errors or mistakes the students commit, it is better to start by defining these two concepts. Firstly, the derivation, we can say that derivation: is the process of forming new word by combining the root which is the basic meaningful unit in English with the affixes, so for example the word (help), it seems clearly that this word is in the basic or smaller situation, therefore we cannot divide it into smaller unit, the word (nation) can be a new word by adding for example the affixes (al), we will have the new word (national). Regarding the inflection: it is the process of combining a root with an affix to indicate the grammatical categories, we mean by grammatical categories the number or the tense etc. For example the word (help), again by adding the affixes (ed) to make the past tense, we will have helped. We believe that the students get confused when differentiating and choosing the correct affix to be added to the root, for example to make the negation or opposition. Even though most of students not only Omani students make this error but it is a concerning field. Types of Mistakes (Derivation) As we mentioned before most of the errors are made by the Arab learner and even Omani one are due to choosing the correct affix to form the negation or opposition, so they rather use "inkind" or "inhappy" rather than unkind and unhappy. Another type of mistake is that some word may end with affixes than aren,t consider real affixes as they are main parts of the word like the words vegetable, syllable, horrible. So the students may think that the root of these words are veget, syll, horr, which makes no sense at all. Source of Difficulty (Derivation) We cannot say the Arabic derivational system is the cause of these mistakes because there is no clear evidence to prove that, and it seems so clear that the English system itself is the cause of the mistakes committed in this area. Another point that needs to be mentioned is that the mistakes made by Arab learners are similar to the ones that are made by students whose mother tongue is not Germanic or Latin origin. Another difficulty is with the affixes that have to work with more than one part of speech, in other words the word reading can be used as a noun as reading is a good way to increase the knowledge, and it is working as a verb like he is reading a book. So in general if there are no rules or generalization of these derivation there is no way to learn them onl