Ancient languages, such as Greek and Latin, which did not use word order for sentence meaning, often specified the role of a noun by changing its form in a sentence. Because nouns can perform a variety of roles, such as showing possession or indicating a subject, these languages had five to six different cases for nouns and pronouns.
Because English orders words to construct meaning, it uses just three noun cases. In two of these cases, subjective and objective, the noun does not change its form. Only in possessive case does the form change by adding an 's to a singular noun or s' to a plural noun to show ownership. Because nouns do not change form for subject and object cases, you have to look at the whole sentence to figure out the noun's role in it. You know a noun is subjective case if it is the subject of the sentence, the doer of the action of the verb. Often, a subjective noun will be the first noun given in the independent clause that is the heart of the sentence. For example, if your sentence is "The boy walks the dog," you know boy rather than dog is the subjective noun because the boy does the action of the verb "walks." Its placement in the sentence also indicates that it is likely the subjective noun.
To find the objective case noun, you must also look at the whole sentence, just as you do to find the subjective case noun. You know a noun is objective case if it is the object of the sentence, the receiver of the action of the verb. To go back to the original sentence, you know that dog is the objective noun because it receives the action of the verb. The dog is being walked by the boy; it is receiving the action of "walks." Likewise, it is the second noun of the sentence, giving the cue that it is likely to be the objective noun.
Pronouns almost always change form for both function and number. Because of this change, it is easier to recognize its role in the sentence if you know which pronouns are subjective and which objective case. For example, the first person singular pronoun "I" is always subjective case, whereas the first person singular pronoun "me" is always objective case. Because the form indicates its role in the sentence, it would not be correct to use me as a subject. "Me walk the dog" is incorrect because me is in the subjective role but it is the wrong pronoun form for that role. "I walk the dog" is correct. If you have not memorized pronoun forms, you can also tell whether the pronoun is subjective or objective by determining which noun it is standing in for. In the sentence, "I" stands in for the subjective noun boy, so you know it too is subjective case and functions as the subject of the sentence.