Here are the completed sentences:

1. Our science teacher is **always forgetting to correct our homework!**
2. I have an appointment to see the doctor at four o'clock.
=> I am **going to the doctor at four o'clock.**
3. The 100-metre runner is about to break her world record!
=> The 100-metre runner is **on the verge of breaking her world record.**
4. I need three months to finish my assignment.
=> In three months, I **will have finished my assignment.**
5. The last time I failed a test was at primary school!
=> I haven’t **failed a test since primary school.**
6. Oliver and William have arranged to visit the planetarium today.
=> Oliver and William are **visiting the planetarium today.**