I know there is Past, Present, and Future tense in English, but what is Imperfect Tense? How do you form it?
The imperfect tense is used to indicate a continuing or repeated action that occurred in the past.
The imperfect tense is typically formed using the present participle (the -ing form of verb) prefixed by "was" or "had been"; however it may be formed using "used to" or something similar with the simple present tense.
I was weeding the garden while my sister watched TV.
Until the day he died, the old man used to rise as soon as the sun touch the morning sky.
The ship had been sailing for three weeks before it reached the Caribbean.