Sentences are composed of phrases and clauses. A phrase is a group of related words that does not contain a subject and a verb. A clause is a group of related words that does contain a subject and a verb. The subject-verb sets are essential units of communication.

There are two types of clauses, and clause type is determined by conjunction type:

Main clauses begin with coordinating conjunctions or none.

Subordinate clauses begin with subordinating conjunctions.

Coordinating conjunctions don't alter the grammatical rank of the words or word groups that they connect:

for, and, but, or, nor, so, yet

Subordinating conjunctions subordinate the clauses they begin, so a clause that begins with a subordinating conjunction is automatically a subordinate clause. "Subordinate" implies dependence on the main clause, suggesting a hierarchy in terms of both grammar and meaning: main ideas in main clauses and subsidiary ideas in subordinate clauses will communicate most clearly. Note that subordinating conjunctions are extremely useful when they're used correctly because they explain how ideas relate.

Here's a partial list of subordinators: after, although, as, because, before, if, once, since, though, unless, until, when, whenever, where, wherever, while.

Relative pronouns may act as subordinating conjunctions: that, which, who, what, whoever, whom, whose.

Subordinators may also be phrases: as if, as soon as, as though, even though, in that, in order that, no matter how, so that.

Adverbial conjunctives (also known as conjunctive adverbs) are often mistaken for conjunctions, but conjunctions create a grammatical link as well as an intellectual one; adverbial conjunctives create only the latter. Thus, they are fine transition tools, but they cannot replace conjunctions in sentence structure and function. This partial list will give you an idea of common adverbial conjunctives: however, therefore, thus, consequently, furthermore.

Main clauses may stand alone as complete sentences, which is why they're also called "independent clauses." When they are combined in a sentence, they must be connected by coordinating conjunctions or semicolons in order to form correct sentences.

Subordinate clauses must be combined with at least one main clause to form a complete sentence, which is why they’re termed "dependent clauses."

Every sentence, therefore, must have at least one main clause.

Run-ons: main clauses without appropriate conjunctions

Fragments: no main clauses

Simple: one main clause

Complex: one main clause, one or more subordinate clauses

Compound: more than one main clause, no subordinate clauses

Compound/complex: more than one main clause, at least one subordinate clause

EXERCISE: Identify the sentence types.

1. Frogs hop.
2. Puerto Rican tree frogs live in bromeliads.
3. Looking for salamanders involves turning over rotten logs, handling numerous slimy things, and frequently getting wet.
4. We found the frogs, but we lost our equipment.
5. Although the fieldwork was exciting, it was also tiring.
6. I know that writing is necessary, but fieldwork is more fun.
7. The current was swift, he could not swim with his pack on.
8. Although he thought he could until that time.
9. Returning once again, to the rainforest, seeking specimens.
10. For example, rare toads, exotic lizards, strange insects.
11. Only a few stars came out, for the moon was bright; the sky was as bright as day, the moon was like the sun, and the stars were unable to compete.
12. Only a few stars came out because the moon was bright even though it was only a crescent.
13. Rainforest soil lacks nutrients and minerals.
14. When the trees are cut, crops cannot be grown successfully.
15. Corporations planning to use the land for grazing are often disappointed, however the rainforest continues to disappear.