Understanding "inflection" and "grammatical category"

Question

The Wikipedia article Morphology says:

A further difference is that in word formation, the resultant word may differ from its source word's grammatical category whereas in the process of inflection the word never changes its grammatical category.

I can partly understand this statement and how "dishwasher" is different from both "dish" and "washer", but I couldn't understand how "the word never changes its category":

• I wash dish. (verb) / He breaks glass.
• A pile of washed dish (adjective) / some broken glass

"Washed" and "broken" are inflected forms of "wash" and "break", but they can be used as non-verb. This is clearly a different grammatical category.

I'm certainly misunderstanding either "inflection" or "grammatical category". What am I missing?

Answer 1

The Wikipedia article means part of speech when it says grammatical category. While the statement is true (with some caveats, participles are counted as inflected, but categorised as verb forms usually, therefore not changing part of speech) for most European languages, both ancient and modern, I am not so sure about other language families, specially the Semitic languages.

Of course, one can define inflection and derivation in a way such that inflection never changes part of speech, but such a definition may be unsatisfactory for other reasons.

The English language has the feature that it can easily change the part of speech of a word without changing the word form itself (To verb a noun and to noun a verb, as a proverb says), this is interpreted as zero derivation in some theories. In English a word form can have different part of speech depending on its usage in a concrete sentence.

Answer 2

What's happening in your example isn't an inflection but a conversion.

Washed is both an inflected form (Past Participle, Present Indicative, Present Perfect, etc all end with -ed) but it's also a participial adjective. The participial adjective is obtained by a word formation process called conversion:

In linguistics, conversion, also called zero derivation or null derivation, is a kind of word formation involving the creation of a word (of a new word class) from an existing word (of a different word class) without any change in form,1 which is to say, derivation using only zero.

In conversion the word can change category (verb to adjective), the inflection is considered to have already happened when the conversion takes place (the inflectional suffix -ed is not considered a derivational suffix).

In terms of morphemes you could consider the inflection to be:

Wash+ed (Verb)

and the conversion to have a zero morpheme:

Wash+ed+∅ (Adjective)

So your examples could be written as:

I wash dish. (verb) / He break+s glass.

A pile of wash+ed+∅ dish (adjective) / some brok+en+∅ glass