Decide vs. Decide (up)onHow to use "decide" with and without a preposition correctly
The verb to decide can be either a transitive verb (a verb that takes an object) or an intransitive verb (a verb that does not take an object unless a preposition follows it).
To decide an issue means “to solve, settle, or determine” it. In this case it is a transitive verb, and no preposition is used. The expression decided on should be used to introduce the ultimate outcome of the situation or decision:
The general’s orders decided on the matter.
The general’s orders decided the matter.
The general decided on/upon a direct assault.
stopped while we decided on/upon which route to take
stopped while we decided which route to take
We finally decided on/upon the quicker route.
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage