Every day, as we use language, we unconsciously select forms of words that feel ‘right’ for the particular ‘slot’ we wish to put them in. When describing someone’s beverage of preference, most English speakers say she drank tea, rather than she drinked tea or some other variant: speakers select a verb form rapidly and will not find alternatives to be acceptable or grammatical.
Occasionally, however, multiple forms may compete within a slot, such as the past participle of the verb prove (have proved? have proven?): here, speakers tend to find both forms acceptable and grammatical, although each of us might only use one of them.
In other places, we lack a suitable form where one is expected: we may baulk at forming the past tense of the verb troubleshoot, where we have a ‘slot’ (past tense needed) but no form that can adequately fill it (troubleshot? troubleshooted?).
These surprising examples of ‘feast’ (multiple forms) and ‘famine’ (no forms) show that selecting or approving the ‘right’ word form is not a process of mechanically mapping one form to a function or vice-versa; instead, speakers may weigh and select forms from a basket of those available to them, sometimes keeping around more forms than necessary, and sometimes failing to find a form that works for them.