In standard English, the formation of preterite and past participle forms of verbs by means of ablaut (for example, sing–sang–sung) is no longer considered productive. Newly coined verbs in English overwhelmingly use the ending -ed for the past tense and past participle (for example, spammed, e-mailed). There are more recent ablaut forms, however—e.g. snuck as the preterite and participial form of sneak, or dove as the preterite of dive. Such cases are standardly said to have been created by analogy instead of by productive application of ablaut rules, though this may to some extent be begging the question. Similarly, the only clearly productive plural ending is -(e)s; it is found on the vast majority of English count nouns and is the almost exclusive means used to form the plurals of neologisms, such as FAQs and Muggles. The ending -en, on the other hand, is (at least relatively speaking) no longer productive, being found only in oxen, children, and the now-rare brethren. In the hacker sociolect, however, the plural -en became at least ephemerally productive for words ending with /-ks/ (on the analogy of ox:oxen), as illustrated by the plurals VAXen, unixen and emacsen, for example.
Productivity is, as stated above and implied in the examples already discussed, a matter of degree, and there are a number of areas in which that may be shown to be true. As the example of -en becoming productive shows, what has apparently been non-productive for many decades or even centuries may suddenly come to some degree of productive life, and it may do so in certain dialects or sociolects while not in others, or in certain parts of the vocabulary but not others. Some patterns are only very rarely productive, others may be used by a typical native speaker several times a year or month, whereas others (especially syntactic processes) may be used productively dozens or hundreds of times in a typical day. It is not untypical for more than one pattern with similar functions to be comparably productive, to the point that a speaker can be in a quandary as to which form to use —e.g., would it be better to say that a taste or color like that of raisins is raisinish, raisiny, raisinlike, or even raisinly? (All four can be found on the Internet.)
It can also be very difficult to assess when a given usage is productive or when a person is using a form that has already been learned as a whole. Suppose a reader comes across an unknown word such as despisement meaning 'an attitude of despising'. The reader may apply the Verb+ment noun-formational process to understand the word perfectly well, and this would be a kind of productive use. This would be essentially independent of whether or not the writer had also used the same process productively in coining the term, or whether he or she had learned the form from previous usage (as most English speakers have learned government, for instance), and no longer needed to apply the process productively in order to use the word. Similarly a speaker or writer's use of words like raisinish or raisiny may or may not involve productive application of the Noun+ish and Noun+y rules, and the same is true of a hearer or reader's understanding of them. But it will not necessarily be at all clear to an outside observer, or even to the speaker and hearer themselves, whether the form was already learnt and whether the rules were applied or not.