Sound interchange is the way of word-building when some sounds are changed to form a new word. It is non-productive in Modern English, it was productive in Old English and can be met in other Indo-European languages.
The causes of sound interchange can be different. It can be the result of Ancient Ablaut which cannot be explained by the phonetic laws during the period of the language development known to scientists., e.g. to strike - stroke, to sing - song etc. It can be also the result of Ancient Umlaut or vowel mutation which is the result of palatalizing the root vowel because of the front vowel in the syllable coming after the root ( regressive assimilation), e.g. hot - to heat (hotian), blood - to bleed (blodian) etc.
In many cases we have vowel and consonant interchange. In nouns we have voiceless consonants and in verbs we have corresponding voiced consonants because in Old English these consonants in nouns were at the end of the word and in verbs in the intervocal position, e.g. bath - to bathe, life - to live, breath - to breathe etc.
Stress interchange can be mostly met in verbs and nouns of Romanic origin : nouns have the stress on the first syllable and verbs on the last syllable, e.g. `accent - to ac`cent. This phenomenon is explained in the following way: French verbs and nouns had different structure when they were borrowed into English, verbs had one syllable more than the corresponding nouns. When these borrowings were assimilated in English the stress in them was shifted to the previous syllable (the second from the end) . Later on the last unstressed syllable in verbs borrowed from French was dropped (the same as in native verbs) and after that the stress in verbs was on the last syllable while in nouns it was on the first syllable. As a result of it we have such pairs in English as : to af`fix -`affix, to con`flict- `conflict, to ex`port -`export, to ex`tract - `extract etc. As a result of stress interchange we have also vowel interchange in such words because vowels are pronounced differently in stressed and unstressed positions.
It is the way of word-building when a word is formed by imitating different sounds. There are some semantic groups of words formed by means of sound imitation
a) sounds produced by human beings, such as : to whisper, to giggle, to mumble, to sneeze, to whistle etc.
b) sounds produced by animals, birds, insects, such as : to hiss, to buzz, to bark, to moo, to twitter etc.
c) sounds produced by nature and objects, such as : to splash, to rustle, to clatter, to bubble, to ding-dong, to tinkle etc.
The corresponding nouns are formed by means of conversion, e.g. clang (of a bell), chatter (of children) etc.
Blends are words formed from a word-group or two synonyms. In blends two ways of word-building are combined : abbreviation and composition. To form a blend we clip the end of the first component (apocope) and the beginning of the second component (apheresis) . As a result we have a compound- shortened word. One of the first blends in English was the word «smog» from two synonyms : smoke and fog which means smoke mixed with fog. From the first component the beginning is taken, from the second one the end, «o» is common for both of them.
Blends formed from two synonyms are: slanguange, to hustle, gasohol etc. Mostly blends are formed from a word-group, such as : acromania (acronym mania), cinemadict (cinema adict), chunnel (channel, canal), dramedy (drama comedy), detectifiction (detective fiction), faction (fact fiction) (fiction based on real facts), informecial (information commercial) , Medicare ( medical care) , magalog ( magazine catalogue) slimnastics (slimming gymnastics), sociolite (social elite), slanguist ( slang linguist) etc.
It is the way of word-building when a word is formed by dropping the final morpheme to form a new word. It is opposite to suffixation, that is why it is called back formation. At first it appeared in the languauge as a result of misunderstanding the structure of a borrowed word . Prof. Yartseva explains this mistake by the influence of the whole system of the language on separate words. E.g. it is typical of English to form nouns denoting the agent of the action by adding the suffix -er to a verb stem (speak- speaker). So when the French word «beggar» was borrowed into English the final syllable «ar» was pronounced in the same way as the English -er and Englishmen formed the verb «to beg» by dropping the end of the noun. Other examples of back formation are : to accreditate (from accreditation), to bach (from bachelor), to collocate (from collocation), to enthuse (from enthusiasm), to compute (from computer), to emote (from emotion) to reminisce ( from reminiscence) , to televise (from television) etc.
As we can notice in cases of back formation the part-of-speech meaning of the primary word is changed, verbs are formed from nouns.