Semper letteris mandate
The principles of grammar are inherently simple. Just as all mathematical operations essentially boil down to adding and subtracting (division is just repeated subtractions, multiplication is repeated additions), grammar boils down to nouns, verbs, and modifiers (considering, for the moment, prepositions and articles as kinds of modifiers).
The most common mistakes come from an inadequate understanding of the relationship between nouns and verbs.
A noun can either be “doing” the action, or having the action “done” to it. In the first case it is called the subject, and in the second case, the object. Understanding the role of subject and object is crucial to understanding grammar.
The good news, however, is that regardless of our level of education, we already have this understanding. We use it, unerringly, whenever we use the words he or him.
The replacement principle
He is subjective, while him is objective. To know which nouns in a sentence are subjects, and which are objects, merely replace each one with either he or him and change the verb to match. When a noun is preceded by an article, such as the, drop the article and just replace the noun. Do the same with words preceded by a possessive, such as his or your.
It sounds more complicated than it is.
Let’s try a simple one to begin:
The cat is on the mat.
Replace the nouns with either he or him, depending on which sounds right to your inner ear. The result is:
He [the cat] is on him [the mat]. [Note that we dropped the article from the mat and just replace the actual noun mat with the word him.]
Since he is subjective, then the word it replaced, cat, must be the subject of the clause, and since him is objective, then the word it replaced, mat, must be the object of the clause.
Now let’s get a bit more complicated.
Peter is going to find the theatre.
Replace nouns with he or him.
He [Peter] is going to find him [the theatre].
Which means Peter is the subject, and theatre the object.
Now look at this one:
Save the cheerleader, save the world.
Save him [the cheerleader], save him [the world].
So…does that mean that there are no subjects? We’ve replaced both nouns with him, which we know is objective. Doesn’t a sentence always need a subject?
Yes — unless it’s “imperative” — which is a type of sentence that gives an order. For instance:
These are all imperative sentences, and the subject in each case is “you.” But the word you is implied, not actually spoken or written.
Let’s try another regular sentence. Remember that one sentence may have several verbs, which means it has several clauses, and each clause must be dealt with separately.
If you don’t finish your homework, I will have to punish you. [Note that we changed the verb to match.]
If he [you] doesn’t finish him [your homework], he [I] will have to punish him [you].
Since each he indicates another clause, and since there are two instances of he in the sentence, this means there are two clauses.
The first clause is, “If you don’t finish your homework.” Replace the you and homework (remembering to drop the your) to get: “If he doesn’t finish him.” So, you is the subject and homework is the object.
The second clause is, “I will have to punish you.” Replace the I and you with he and him to get: “he will have to punish him.” So in this case, the you is the object, and I is the subject.
There are of course, sentences for which this trick just doesn’t work, but in general it’s a pretty good guide. Best of all, after a while you start to intuitively feel the subject and object, even without replacing any words.