Verbs are constantly changing, either to indicate changes in tense or to become other parts of speech. In the correction, we add -ed to explain to complete the passive voice (verb to be + past participle). Also, study becomes studying. When we add -ing to study, we create a gerund (a verb made into a noun) that serves as the object of the preposition by.

Check the following resources for useful practice material: 1) Guide to Grammar and Writing, 2) Colleen Hildebrand's UCI Correction Symbols with hyperlinks.

For more information on verbs and how they change, see the LARC Electronic Writing Center Grammar Checklist.