You may know that English is an infected language—i.e., a potpourri of many influences, particularly Germanic tongues, Latin, and French—and is as well an infectious language, as it has become the recognized lingua franca of the modern world.  What you may not know is that English is also an inflected language.  This means that the form of some words are altered to indicate their grammatical status or their relationship in a sentence.  This is really less complicated than it sounds, and a clear awareness of what is meant by "case" may assist you in speaking and writing correctly, at least as it affects pronouns.

First, however, let us take a look at inflection as it relates to verbs, which is termed conjugation.  This refers to the way we alter the form of a verb, depending on the number, person, and tense.  For instance, "He runs for the ball," but "yesterday they ran six miles."  These changes pose little problem for most folks, though you may occasionally hear someone say, "If I had went there…."  Knowing the principal parts (present, past, and perfect tenses) of so-called "strong verbs," or irregular verbs, helps avoid this mistake.  (Most dictionaries provide these, and Diana Hacker's A Pocket Style Manual (4^th ed.),which we require in Historical Writing and Senior Seminar, has a list of some of these on pp. 30-32.)

By far the trickiest verb for most English-speakers appears to be "to lie" (meaning to recline, not to prevaricate); the second most difficult is the one so frequently confused with it: "to lay" (to put).  Yet a simple mastery of the principal parts of these verbs can solve the problem:

Therefore, "I regularly lie on the couch in the afternoon"; "Yesterday I lay down on it for an hour"; "For a week now I have lain down in order to take a nap."  "Lain down" may sound odd, but it is correct.  Notice two issues with these sentences: 1) the verb "to lie" takes no direct object (i.e., you cannot "lie" something; instead the verb requires a preposition, such as "on"), which means it is an intransitive verb; and 2) "down" so often following "lie" tends to lead one to hear "laid" instead of the correct "lay" or "lain."

"To lay" is much simpler.  The form of this transitive verb (meaning it takes a direct object) is the same for all past and perfect tenses ("laid"), so one need only remember that it is "lay" in the present tense: "I lay the book on the table whenever I tire"; "Yesterday I laid the book on the table and forgot I'd put it there"; "I have laid the book on that table so many times that it's hard to imagine I forgot where I'd put it."

If conjugation is for verbs, then declension is for adjectives, nouns, and pronouns (i.e., those little words, such as "he," "she," and "it," that substitute for nouns).  Fortunately for us, English declines (or alters the form of) only pronouns.  Before one can fully appreciate how this works, however, it's essential to know something about "case."  Case refers to the grammatical function of the pronoun in a sentence.  The pronoun might be the subject of the sentence ("He gave the book to me"), or it might be the direct object ("He gave it to me"), or the indirect object ("He gave it to me"), or even the possessive case ("He gave my book to her").

Here's a helpful layout of pronoun cases:

This seems easy enough; the problem, however, often arises with so-called "compound word groups," that is, when a pronoun is tied to at least one other subject or object.  No one would say, "Me is going to the movies," but far too many say, "My brother and me are going to the movies."

Whereas understanding case is important in analyzing a pronoun in a sentence in order to determine its correct form, for the most part the advice my Grandpa Allen gave me ages ago does the trick: simply strip the sentence down to the pronoun you're unsure of.  In the example, "My brother and me are going to the movies," eliminate "My brother and," and the need for "I" rather than "me" becomes clear.  Here's a more challenging example: "Bob gave the books to her and me."  Though the tendency for some is to think it's always correct to say "she and I," this holds true only in the subjective case.  In the above example, the two pronouns are in the objective case, receiving rather than giving, and therefore the sentence must read "her and me."  Likewise, "He gave the books to Bob and me" is more obviously correct if you take away "Bob," making it easier to see "me" is needed rather than "I."

Were I to offer further instruction in pronoun inflection, you'd undoubtedly decline (that's a pun), but this covers the basics of this topic.  For further discussion of related situations that might make one pause, see Hacker, pp. 40-44.  After a look at that you'll be a pro regarding "who" vs. "whom"!  Also, if you REALLY want a handle on all this case stuff, just take a German or Latin course; that will really do the trick!