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In English language, there are four main criteria which are used to make a distinction  between derivational and inflectional morphemes.  They are: productivity, order, ability to change category and  relation to grammatical category and syntax.

1. Productivity

Inflectional suffixes attach to roots/stems in a very predictable way and they can be added to almost all words in a particular grammatical category. For example, count noun chosen by chance, almost always  allows an inflected plural. On the contrary, an arbitrarily chosen derivational affix applies only to a restricted class of bases.  In order to prove this, I have found twenty-seven examples of inflected plural nouns on only one page and only one derivated adjective which has derivational adjective suffix less on the same page. Inflected plural nouns are: years, pounds, preferences, folks, valedictories, discourses, misings, listeners, aromas, pillows, biscuits, sleeves, tables, dreams, sleepers, windows, timbers, canticles, tides, breakers, islands, workers, brothers, nets, mariners, eyes, and  efforts. (Conroy, 2005:295) The only derivated word that I have found on the same page is an adjective helpless. (Conroy, 2005:295)  Despite the fact that derivational adjective suffix less is quite common in English language, it typically occurs with only some members of a class of morphemes. For examle, we cannot say happyless or beatyless.  Inflectional morphemes are very productive and they typically occur with all members of a class of morphemes (for example, the plural morpheme s occurs with almost all nouns). On the other hand, derivational morphemes are usually not very productive and they typically occur with only some members of a class morpheme.

2. Order

Derivational morphemes occur before any inflectional suffixes are added. They have to be added to the base before any inflectional affixes –reminds -(Conroy, 2005:324)  But,   inflectional affixes are attached to the base after any derivational affixes. In other words, they occur at margins of words, after any derivational morphemes – reminds- (Conroy, 2005:324)

3. Category change

Inflectional affixes do not change the category of the base to which they attach –boy-boys – noun - (Conroy, 2005:294) However, derivatinal affixes can change the category of the base to which they attach – hope (noun) – hopeless (adjective) - (Conroy, 2005:166).  Nevertheless, derivational affixes do not necessarily change the base to which they are added – place (verb) – replace (verb).  (Conroy, 2005:25)  I have to point out that inflectional ing is different from derivational ing. Inflectional ing is added to verb to form progressive tenses: I was thinking about time the other day. (Conroy, 2005: 323)  Derivational ing is attached to verbs to form adjectives: There was a disturbing apposition to the American lives we were leading. (Conroy, 2005:547) Besides, there is the difference between inflectional ed and derivational ed. Inflectional ed is attached to verbs to form past tenses: In Colleton I had entered into the teacher`s life of sustained regularity. (Conroy, 2005: 549)  On the other hand, derivational ed is added to verbs to form adjectives: He was surprised and embarassed by the fuss. (Conroy, 2005:476)

4. Relation to grammatical categories and syntax

Inflectional affixes are categories that are relevant to stating the syntatic rules of the language. Because of that, they are obligatory to the rest of the sentence in which they occur in a way that derivational affixes are not.

She thinks you`re going to lose your shirt again. (Conroy, 2005:302)   We can never say: She think you`re going to lose your shirt again.

Suan said you were displeased when you found out that I was coaching him. (Conroy, 2005:506)   Susan said you were pleased when you found out that I was coaching him.- Although the meanining is changed, the sentence is grammatically correct.

            Besides these four major differences, there are other differences as well:

1. The dictionary usually lists derivative lexemes, but not inflected words. Inflected words are rule-regulated and that`s why regularly inflected forms are usually not listed in dictionaries. Derivatioanal affixes can change both the syntatic category and the meaning and because of that, they appear in dictionaries with derivational morphemes.
2. Derivational morrhemes may be prefixes and suffixes, but inflectional morphemes are all suffixes in English. English inflectional suffixes are always final in the morpheme groups to which they belong.  
3. Derivational morphemes typically indicate semantic relations within a word, while inflectional morphemes typically indicate syntatic or semantic relations between different words or sentences.  CORPUS – Conroy, Pat. The Prince of Tides. (2005). New York: The Dial Press.