Where can I find a database of English words that has the various forms for each word? Specifically, it would give the plural and singular form with its indefinite article for each noun, the various forms, tenses, and voices for verbs, and the comparative and superlative for adjectives. It would be easy to use it to solve queries like (noun,plural,"fish") and (verb,past,"to go").
Have a look here, maybe there's something useful among those tools: http://aclweb.org/aclwiki/index.php?title=Morphology_software_for_English
Technical note: A "database" would be a less than ideal format for a language's morphology given the high redundancy among different word forms (all with the same stem); one typically uses a finite state machine/transducer for that.