Mechanics

Sentences are like the cells in a human body: individually, they are small and self-sustaining, but when they group together, they can make everything from a bacterium to a whale. Each cell has an individual role to play and even forms in a specific way to perform a specific action. Nerve cells carry information. Blood cells transport oxygen. Bone cells support bone structure.

All cells function to support the whole body... or should. Sometimes cells don't do their jobs or are broken: this becomes cancer. Cancerous cells are abnormal, and to determine the cause of the abnormality, scientists focus on the proteins and nucleic acids that caused the cell to mutate and become broken. They look at the how the smaller pieces affect the whole. This is the same mindset that writers have when thinking about grammar.

If a sentence "isn't working," you need to look smaller: analyze the clauses and phrases.

Just as abnormal cancerous cells can affect the whole body, an abnormal sentence can impact the meaning of the sentences around it, and enough of these bad sentences can make the entire work weak. Just like with the cell, abnormal and grammatically incorrect sentence cannot be fixed at the sentence level-- one must break down its phrases and clauses to diagnose why the sentence is bad. Good sentences are made of good phrases and clauses working together to create meaning. So what are phrases and clauses?

Clauses are units of subjects and predicates.
Phrases are a group of words revolving around an object.

CLAUSES

Subjects and verbs

All clauses have a subject and a predicate verb. A subject is who or what the clause is about; subjects are always nouns or pronouns, and are usually the first noun or pronoun in the sentence. Any word that modifies the subject is part of the extended subject. For example, in The bright light hurt my eyes, light is the subject and The bright light is the extended subject.

Every subject must be paired with a verb that describes what the subject is doing or how it is. This verb is called the predicate verb, and while the predicate of a clause always consists of at least this one verb, it could encompass a complement and any words that modify that verb or complement. For example, in The bright light hurt my eyes, hurt is the predicate verb, while hurt my eyes is the extended predicate.

So how do subjects and verbs pair? Through a concept called subject-verb agreement:
ACTION VERBS:

If the subject is singular, then the present verb is plural: The dog walks. (dog is singular, walks is plural)
If the subject is plural, then the verb is singular: The dogs walk. (dogs is plural, walk is singular)
If the verb is not present tense, the subject's plurality is not important: The dog walked, The dogs walked, The dog will walk, The dogs will walk
If there are two singular subjects connect by and, treat as a plural: The dog and cat walk
If there are two subjects, one plural and one singular, treat as a plural: The dogs and cat walk

LINKING VERBS:

If the subject is singular, then the verb must be singular: A dog is hungry. (dog and is both singular)
If the subject is plural, then the verb must be plural: The dogs are hungry (dogs and are both plural)
Plurality is important if verb is past or present with aspect: The dog was hungry, The dogs were hungry, The dog is walking, The dogs are walking
If the verb is future tense, plurality is not important: The dog shall be hungry, the dogs shall be hungry
If the subject is a collective noun, plurality depends on context: The pack is ready, the majority are hungry

Complements

A complement is a word that completes the idea of a subject and verb. There are four types of complements:

DIRECT OBJECT: a direct object is an object (noun or pronoun) affected by the verb action. In I threw the ball, ball is the direct object as it is being thrown. When an action verb takes a direct object, it is called a transitive verb. When it takes more than one direct object, it is a ditransitive verb. If the verb does not take a direct object, it is an intransitive verb.
INDIRECT OBJECT: an indirect object is an object (noun or pronoun) that receives or benefits from the direct object. In I threw Michael the ball, Michael receives the ball and is thus the indirect object.
SUBJECT COMPLEMENT: a noun or adjective which renames or modifies the subject. There are two subject complements: predicate adjectives are modifiers that describes the subject (Batman is dedicated to justice); predicate nominatives are nouns or pronouns that renames the subject (Batman is secretly Bruce Wayne.)
OBJECTIVE COMPLEMENT: a noun or adjective which renames or modifies a direct object, often used with verbs like make, name, call, elect, and appoint. In The twins kept their room clean, clean is the objective complement.

Types of clauses

If a clause makes sense on its own, it is called an independent clause. EVERY SENTENCE HAS AT LEAST ONE INDEPENDENT CLAUSE. In the sentence When the world is in trouble, Superman will always be there to save it, the green clause is independent.

If the clause doesn't make sense on its own, the clause is a dependent clause. A dependent clause can act as noun, an adjective (called a relative clause), or an adverb:

NOUN CLAUSE: a clause that functions as a subject, direct object, or object of a preposition (What Superman wants is justice).
RESTRICTIVE RELATIVE CLAUSE: begins with a relative pronoun, is necessary for the meaning of the sentence, and functions as an adjective (The man that married my aunt is a circus clown).
NONRESTRICTIVE RELATIVE CLAUSE: begins with a relative pronoun, is not necessary for the meaning of the sentence, and functions as an adjective (That man, who is a circus clown, married my aunt).
ADVERBIAL CLAUSE: a clause that starts with an adverb and clarifies where, when, or why something happens (Superman was defeated because Lex Luthor had kryptonite).
ELLIPTICAL CLAUSE: a noun, relative, or adverbial clause that is grammatically incomplete but makes sense in context (Although once common, dodos are now extinct).

PHRASES

A phrase is a string of grammatically related words that do not form a full clause.

NOUN PHRASE: a noun and its modifiers (The bright green tennis ball fell from the sky).
VERB PHRASE: The main verb and all of its helping verbs (Cyndi will have been throwing up for two days).
PREPOSITIONAL PHRASE: a phrase starting with a preposition and ending with the object of the preposition (This sentence has three prepositional phrases: Marvin slept in Mary's bed last Thursday by accident).
APPOSITIVE PHRASE: an appositive is a noun that renames the noun immediately before it (Batman’s butler Alfred raised him after his parents Thomas and Martha Wayne died.) Note that simple appositives don't require commas. An appositive phrase is an appositive with modifiers that does require commas (Alfred, a former British intelligence officer and one-time actor, became Batman's first ally in his war against crime.)
VERBAL PHRASE: uses a verbal with an object
- INFINITIVE PHRASE: begins with an infinitive: Batman hopes to stop the Joker’s plan in time. In this example, plan is the object of the infinitive.
- GERUND PHRASE: begins with a gerund: Fighting crime as Batman arguably is what keeps Bruce Wayne sane. In this example, crime is the object of the infinitive.
- PARTICIPLE PHRASE: begins with a participle: Removing his mask, Batman revealed his true face to Catwoman. In this example, mask is the object of the infinitive.

CLASSIFYING SENTENCES

Classifying through clauses

SIMPLE: a sentence with a single independent clause (Batman and Superman are friends.)
COMPOUND: a sentence with two or more independent clauses (Superman is a super-powered alien, but Batman is just a regular man in a costume.)
COMPLEX: a sentence with a single independent clause and one or more dependent clauses (While Superman can only be weakened by magic or kryptonite, Batman has normal human weaknesses.)
COMPOUND-COMPLEX: a sentence with two or more independent clauses and one or more dependent clauses (Not only are Batman and Superman friends, but they have are friends with other heroes, who together from the Justice League.)

Classifying through purpose

DECLARATIVE: a statement of fact or opinion (Aquaman is a terrible hero.)

Periodic: Puts the clause with the main idea at the end (Despite his birth on land, Aquaman is still "king" of the seas.)
Cumulative: Puts the clause with the main idea at the front (Everyone thinks Aquaman sucks--both heroes and villains mock him.)
Balanced: all clauses have equal emphasis (Batman "forgets" to call him to battle, and Flash never texts him back.)

INTERROGATIVE: a direct question (Does anyone even like Aquaman?)

Indirect: starts with a model verb (Could you tell me how you feel about Aquaman?)
Direct: starts with a non-model or interrogative pronoun (What does Aquaman add to the Justice League anyway?)

IMPERATIVE: a demanding order or command (Let's all boycott Aquaman's inclusion in the Justice League!)

Horative: advise encouraging a course of action (Aquaman, you should go away and leave the heroics to Superman.)

EXCLAMATORY: an urgent, excited statement (Oh no, it's Black Manta! Save me, Aquaman!)

Classifying by formula

Early 20th century grammarians created a list of five essential sentence formulas that create a simple one-clause sentence. Note that any phrases can be added to the frames to make them more complex, but all simple sentences ultimately conform to one of these five formulas:

subject + intransitive verb (Ella complains.)
subject + intransitive verb + subject complement (Ella seems frustrated.)
subject + transitive verb + direct object (Ella hates Katie.)
subject + transitive verb + direct object + object complement (Ella made Katie suffer.)
subject + ditransitive verb + indirect object + direct object (Ella shot Katie an evil look.)

Sentence Mechanics