Initial observations suggest that the terms “something” and “some thing” have only one significant difference: The first term is written as one word, while the latter is written as two words. While this is true, the more pertinent differences are how the two terms are properly used in writing.
How to Use "Something"
“Something” written as one word is an expansion of the pronoun “some.” “Something” is used as a pronoun to take the place of a noun, usually solidifying the relationship of the noun to the rest of the sentence. “Something” as an indefinite pronoun functions uniquely because this term can be modified by adjectives, or descriptive words, following the pronoun, whereas adjectives typically precede nouns and pronouns. For example, “Something sweet was left on the counter.” “Sweet” describes something in this sentence. Other nouns, clauses, verb phrases and prepositional phrases may also modify the pronoun “something.”
How to Use "Some Thing"
“Some thing” written as two words should be viewed as a noun phrase, in which “some” is an indefinite pronoun modifying the noun “thing.” This phrase typically appears in nominal -- or noun-like -- positions, such as the subject, direct object, indirect object and predicate nominative slots. For example, “Some thing is making noise in the closet.” “Some” is modifying the noun “thing,” which is positioned as the subject of the sentence. At times, “some thing” may be written in the plural form by adding an –s to “thing.” For example, “I’m still learning some things about this job.”