The sentence can be correctly completed as follows: 'I saw a strange-looking fellow yesterday. I don't think I had seen him before.' Both verbs are representing actions that occurred in the past.
The completion of your sentence relies on the correct use of past tense verbs. The verbs needed to complete the sentence are 'saw' and 'had seen'. The correct sentence will be: 'I saw a strange-looking fellow yesterday. I don't think I had seen him before.' This verb tense is used because the actions you are talking about were in the past.
The verb 'saw' is the simple past form of the verb 'see', while 'had seen' is using the past perfect form. The latter is utilized to indicate that one action in the past happened before another past action, in this case, you observing the strange-looking fellow happened before you recalling whether you had previous interactions.
Learn more about Past Tense Verbs