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From LogicWiki

A declarative sentence is a piece of language that can be either true or false. (Or, a piece of language capable of being either true or false. "Donald Trump will be President in 2016" is a declarative sentence (capable of being either true or false--probably false!).) This is to contrast it with other pieces of language that aren't true or false, like questions or commands. In logic, we confine ourselves to the use and analysis of declarative sentences only, ignoring these other pieces of language (except in very specialized subspecialties that need not concern us here). Finally, we will take the word statement to be interchangable with sentence, though "sentence" will typically carry the day. (I.e., it is fine to say that "Irvine is a city" is a declarative statement.)

Examples:

"San Francisco is a city in California" is a sentence, and a true one.

"The atomic number of Hydrogen is 17" is also a sentence, but a false one.

"Answer the phone!" is not a sentence, it is a command. It does not make sense to say that "answer the phone!" is true or false.

"Is that Brian?" is not a sentence, it is a question. It does not make sense to say that "Is that Brian?" is true or false (it does make sense to say that "it is Brian" is true or false, because "it is Brian" is a sentence).

For a more indepth analysis of sentences and their structure, see Propositional Logic and Predicate Logic.

Atomic vs. Compound

An atomic sentence is a sentence that contains no (proper) parts that are in turn sentences. For instance John runs is an atomic sentence because, although it has parts (the singular term John and the predicate runs), these are not, in turn, sentences.

Examples of Atomic Sentences

*John runs.
*Mary jogs.
*Cake is good.

A sentence that is not atomic is a compound sentence:

A compound sentence is a sentence having parts that are themselves sentences. For instance, this is most often achieved through the use of logical connectives (i.e., expressions that allow us to obtain new sentences from old ones).

Examples of Compound Sentences

*The ocean is calm or the sky is cloudy.
*Bill went to the movies and Mary stayed home.
*Bill didn't stay at home.

See Also

Tautology, Contradiction, Contingency.