I'm looking for a maximum subset of the English language such that words can be divided into two categories, one of which must include at least nouns, proper names, adjectives, numerals, verbs and adverbs, and the other must include at least the articles ('a', 'the'). Each of the groups may, and should, include other words, like different kinds of pronouns, etc.
It is OK to omit some words if they cannot be placed in one of the two categories (sacrificing the expressive power of the resulting subset of English). For example, if "will" the noun and "will" the verb, or "mine" the noun and "mine" the possessive pronoun fall into different categories, they may be omitted (yes, even sacrificing the whole future case is tolerable).
It wouldn't be hard if there were no constraints, would it? The division has to be natural (intuitive for a native speaker; words in the first group should be considered substantially more "significant" than words from the second one) and unambiguous: no word with the same spelling should fall into both categories, regardless of position in the sentence, part of speech, etc.