What is the main process of word formation in the English language?
The main process of word formation in English is derivation, which is a category of affixation. Inflection is the second category of affixation but does not move a word from one word class to another word class or from one syntactic category to another syntactic category as derivation does, thus is not a process in word formation.
Derivation is the formation of words by the affixation of suffixes and occasionally prefixes that have a syntactical influence and cause a syntactic change in the word. For instance, the affix -s does not change the word class or syntactic category of shoes. Thus this affixation is inflecional rather than derivational. Derivational affixes, in particular suffixes, are those that form syntactical changes such as that of forming, as examples, a noun from a adjective (-ness) or that of forming a verb from a noun (-fy) or that of forming an adjective from a verb (-able) or that of forming an adverb from an adjective (-ly).
Most prefixes change the meaning of the word without causing a syntactic category change. For example, the meaning of do is made negative by un- in undo yet both are verbs. Nonetheless, there are some prefixes that are word forming such as en-/em-, which may form a transitive verb from an adjective or a noun. Similarly, be- may form a transitive verb from a noun as in the noun token forming the transitive verb betoken. [For more detail see "Word Formation Processes in English" by Mohsen Yousefi. Fars Science and Research University.]
English is a Germanic based language rather than the Latin based languages of Spanish, French or Italian. English is also a language which borrows words whole from other languages such as 'coup' from French which literally means 'a blow.' Suffixes have changed in how they are used such as 'ette' which used to mean 'diminutive' and now has changed to creating a word like majorette while not losing the former idea of small. English also adds words each year and the process is increasingly fast. Words which deal with computer technology like interface which means the computer can communicate with other computers or the user have been added. Because English has become a world language, words in each country have evolved in their own context such as a 'cookie' in America is a 'biscuit' in England. I'm not sure if this is exactly what you were looking for, but I hope it helps you with your question.