studied
v.v. to spend time learning about a subject by reading, practicing, or going to school. You do this to gain knowledge or prepare for an exam.
v. to devote time and attention to acquiring knowledge on an academic subject. Transitive when followed by a subject; intransitive when describing the general activity.
She studied hard for her math test yesterday.
After working all day, he studied law at night to prepare for his career change.
Researchers studied the behavior of the local bird population for three years before publishing their findings in a peer-reviewed journal.
When used as a transitive verb, it takes the subject of learning as a direct object.
I studied about historyI studied historyThe verb is transitive when naming a subject; it does not require the preposition 'about'.