Sure! Here are the completed sentences with the correct forms of the given words:

1. He's a really **reliable** person.
2. She made several excellent **observations** in her essay on Charles Dickens.
3. Watching TV shows in English is definitely very **useful** to improve your listening skills.
4. The Internet is probably one of the best **inventions** ever.
5. The boat was 16 feet in **length**.
6. Both **accuracy** and fluency are important when speaking a foreign language.
7. You shouldn't have said that! It was a totally **inappropriate** remark.
8. Since the earliest times, civilisations have understood the **importance** of time.
9. Doing puzzles keeps our brains fit and **healthy**.
10. As well as gaining **satisfaction**, by doing puzzles we give our brains a good workout.