The past tense of telecast is telecast.
Will be telecasted is the natural form, although will be telecast is common. Telecast comes from broadcast, which comes from cast, for which the past participle is casted. However, unobservant speakers frequently mistake the -t in cast for an allomorph of the past marker -ed.
The past participle of "do" is "done." The past participle of "have" is "had."
The past participle of "am not" is "have not been."
The past participle of "do" is "done."
The past tense of "have" is "had," and the past participle is also "had."
The past participle of the word "have" is "had."
The past participle of "am, are, is" is "been."
The past participle of "will" is "willed."
The past participle is thought.
The past participle of "to" is "been." The past participle of "be" is also "been."
Being is the present participle. The past participle is been.