I was watching a TV program, when he ______ in.
I was watching a TV program, when he ______ in.
Explanation
I was watching a TV program when he walked in.
The past tense "walked" is used to match the past continuous tense of the main action.
I was watching a TV program when he walked in.
The past tense "walked" is used to match the past continuous tense of the main action.