The subject of a sentence or clause is the noun(s) or pronoun(s) (plus any related words, such as adjectives) that fulfills the action articulated by the verb.
In active-voice clauses, the subject is performing an action (doing something), whereas in passive-voice clauses, the subject is receiving an action (having something done to it).
Either way, it's important to select subjects carefully. Readers generally take what comes at the beginning of a sentence—which is usually the subject—to be what the sentence is "about." Therefore, readers find writing to be clearer and more concrete when the subject is precise, which also tends to make the verb more precise.
Compare these two statements (subjects are underlined, verbs are in italics): which is more forceful?
- There are three ways in which diplomacy can help in such situations: . . . (In this common but grammatically inverted construction, "there" is an adverb that appears to stand in for the subject.)
- In such situations, diplomacy can help in three ways: . . .
Most readers will experience the second sentence as the more straightforward of the two. The first version uses the placeholder "there are," which "locks" the important action—helping—in a relative clause. In the second version, the subject and verb reflect what the sentence is actually about.
Moreover, using a precise subject and verb here produces good flow: it allows the transition ("In such situations") to move up front, where it usefully contextualizes this statement, and the new information being introduced ("three ways") to move to the end, where it leads us to the next idea—i.e., the three ways.
Using accurate subjects conveys your meaning more clearly to the reader and can help many other parts of the sentence fall into place, usefully propagating that original act of grammatical attention.