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Do you mean “weak” in the sense of having weak language?
[list]
[]Remove unnecessary words from your writing (if you can remove a word without changing the meaning of the entire sentence, it’s unnecessary). For example, in the sentence, “The room was completely empty,” “completely” is an unnecessary word.
[]You want to write as if you’re having a conversation with a respected adult (like a teacher). Care about your grammar and spelling, but don’t be overly formal. Stick to words you use in everyday speech (no “SAT vocab!”). Fragments, starting sentences with conjunctions or ending them with prepositions, etc. are fine in moderation.
[]“Show,” don’t “tell.” Instead of writing, “I was very embarrassed,” write, “I shuffled my feet, unable to look anyone in the eye. I hoped that no one could see how hard I was blushing.”
[]Short, concise sentences are always better than run-on ones. Read your essay out loud to gauge whether a sentence might be too long.
[*]Avoid passive voice (e.g. instead of “We were given $100 by a generous donor,” write, “A generous donor gave us $100.”).