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When words like “I believe” and “I think” begin a sentence, they are functioning as the subject and verb in the sentence and take no punctuation.
…I believe he called in a little before 3:00.
…I think he was with us for ten years.
When they are in the middle of the sentence, they are parentheticals and need commas around them.
…He called in, I believe, a little before 3:00.
…He was with us, I think, for ten years.