At the end of a complete sentence we normally put a full stop (or a question mark, or an exclamation mark). Please remember that a complete sentence is a sentence that minimally has a subject and a predicate verb, so (1) is an example of a complete sentence that contains nothing but the two obligatory constituents:
(1) She sings.
We may also join two or more sentences by using a coordinating conjunction, such as and, or, but, or so. Sometimes a semicolon can be used between two sentences, to signal that the two sentences are closely and logically connected to each other. A run-on sentence is when two or more complete sentences (independent clauses) follow right after each other, without a full stop (question mark or exclamation mark), a semicolon, or a coordinating conjunction. Often a comma is used in such run-on sentences, but separating two sentences is not one of the ways in which commas can be used (follow the link below to see how commas should be used).
In (2) we have an example of a run-on sentence:
(2) The lecturer marked all the exams in record time, he handed them out the same day.
In (2), there is nothing but a comma separating the two complete sentences, but separating two sentences only using a comma is not acceptable (in formal/academic writing). There are four different solutions that can be used to avoid such run-on sentences:
- Write the two sentences (independent clauses) as two separate sentences (see example 3 below).
- Insert a coordinating conjunction preceded by a comma between the two independent clauses (see 4 below).
- Insert a semicolon between the two independent clauses (as in 5 below).
- Insert a semicolon and a linking word between the two independent clauses (as in example 6 below).
(3) The lecturer marked all the exams in record time.He handed them out the same day.
(4) The lecturer marked all the exams in record time, and he handed them out the same day. (5) The lecturer marked all the exams in record time; he handed them out the same day.
(6) The lecturer marked all the exams in record time; therefore, he handed them out the same day.
