1. My bicycle is not here. Somebody has taken it.
2. Last night I arrived home at half past twelve.
3. David thinks of his plans again and again.
4. I remembered that I had given Simon my pen.
5. They had written a letter by 5 o'clock. или They will have written a letter by 5 o'clock.
6. He told me he had often thought of it. или He told me he often thought of it.