Start a sentence collection. Memorize about 20 sentences that have good structure, and use variations of them. For example:
School uniforms are useful, but they have serious drawbacks.
You can make variations of that sentence. If you are writing about capitalism, you can write:
Capitalism is powerful, but it has serious drawbacks.
or you can change it in a big way:
Capitalism is a sophisticated system that mirrors natural selection from evolutionary theory, but it also creates greed and ruthlessness.
Try making variations of this sentence:
When the sky gets dark, the world is trying to go to sleep.
ANOTHER IMPORTANT IDEA:
Post some more writing here and ask specifically with help conjugating verbs. That is usually what people need the most help with. You are doing well! I bet if you took the test again right now you would score higher. They said you insufficiently answered some questions... so maybe you need to write more than you wrote.
Post an essay soon! We'll practice....