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English has two basic ways of combining words into groups: phrases and clauses.
- Phrases are centred around nouns (in the van, by early morning).
- Clauses are centred around verbs (she runs the marathon; when he saw the ruins).
Types of clauses
All sentences are constructed from two types of clauses.
- The main, principal, or independent clause. It contains a subject (noun), a predicate (verb), and expresses a complete thought:
She decided to walk to the park.
(subject) (verb) (= complete thought)
- The subordinate or dependent It also contains a subject and a verb but does not express a complete thought. Subordinate clauses often begin with words like since, while, although, despite, etc.:
Because it was a sunny day…
(subject) (verb) (= incomplete thought)
Combining a subordinate and a main clause
Combining clauses is what sentence building is all about. Joining a subordinate clause with a main clause requires only a comma:
✓ Because it was a sunny day, she decided to walk to the park.
Combining two main clauses
The tricky part comes when two main clauses are joined together. Punctuation options include:
✓ A period: It was a sunny day. She decided to walk to the park.
✓ A semi-colon: It was a sunny day; she decided to walk to the park.
✓ A comma with a linking word (coordinating conjunction): It was a sunny day, so she decided walk to the park.
Other coordinating conjunctions are and, but, yet, or, nor and for.
✘ Comma splices: Do not use a comma to join two main clauses in a sentence: e.g., It was a sunny day, she decided walk to the park. A comma is too weak a form of punctuation, and the resulting error is called a comma splice.
✘ Run-on sentence: Joining two main clauses with no punctuation is called a run-on sentence.
Both are considered to be major sentence errors.
A special case is when two main clauses are combined, and the second one begins with however (or other conjunctive adverbs like therefore, thus or nevertheless). A semi-colon must be used; a comma is too weak, and will create a comma splice:
✓ It was raining; however, she decided to walk to the park.