Robert McColl Millar

“the highly complex patterns of plural formation in Old English have been replaced almost entirely by the iconic -s plural, which has spread to many hundreds of words that formerly didn’t have it.”
― Robert McColl Millar, Trask's Historical Linguistics

“Another, much more famous, instance of morphologization has occurred in the Romance languages (the modern descendants of Latin). Latin had a noun mens ‘mind’, whose stem was ment- and whose ablative case-form was mente (the Latin ablative was a case-form with miscellaneous uses, most of which we would associate largely with prepositional use in English). Quite early, it became usual in Latin to use the ablative mente with an accompanying adjective to express the state of mind in which an action was performed; as was usual in Latin, the adjective had to agree with its noun mente as feminine singular ablative. We thus find phrases like devota mente ‘with a devout mind’ (i.e., ‘devoutly’) and clara mente ‘with a clear mind’ (i.e., ‘clear-headedly’). At this stage, however, the construction was possible only with adjectives denoting possible states of mind; other adjectives, like those meaning ‘new’ or ‘equal’ or ‘obvious’, could not appear with mente, because the result would have made no sense: something like ‘with an equal mind’ could hardly mean anything. But then speakers began to reinterpret the mente construction as describing not the state of mind of somebody doing something, but the manner in which it was done. Consequently, the construction was extended to a much larger range of adjectives, and new instances appeared, like lenta mente (lenta ‘slow’) and dulce mente (dulce ‘soft’), with the adjectives still in the appropriate grammatical form for agreement with the noun. As a result, the form mente was no longer regarded as a form of mens ‘mind’; it was taken instead as a purely grammatical marker expressing an adverbial function, and it was therefore reduced from a separate word to a suffix. Today this new suffix is the ordinary way of obtaining adverbs of manner in the Romance languages, entirely parallel to English -ly in slowly or carefully, and it can be added to almost any suitable adjective. Thus Spanish, for example, has igualmente ‘equally’ (igual ‘equal’) and absolutamente ‘absolutely’ (absoluta ‘absolute’). Spanish still retains a trace of the ancient pattern: when two such adverbs are conjoined, only the last takes the suffix, and hence Spaniards say lenta y seguramente ‘slowly and surely’, and not *lentamente y seguramente. In French, this is not possible, and a French-speaker must say lentement et sûrement.”
― Robert McColl Millar, Trask's Historical Linguistics