Thanks for getting in touch. I believe that these are very important grammar questions and getting them correct will help you score higher in OET writing as well as speaking.
Unfortunately, we cannot answer students' grammar questions on this platform.
However, I encourage you to use any of these resources to improve grammar.
1. Online self-learning course. Please login to your student platform and go to Core Skills. All grammar lessons can be accessed there.
2. There are a number of grammar video lessons in the OET course that you can refer to.
3. In addition, you could discuss your grammar in a tutorial with one of our expert teachers.
Please let us know if you have more questions that we could help with.
All the best