How do I improve my expression?
Your expression is not “sophisticated” just because you may be writing longer sentences. On the contrary, often awkward or clumsy expression occurs because you have too many clauses in one sentence, or your phrases or clauses do not have a clear subject-verb combination. If the sentence has two clauses each with a clear focus then this is better than longer and clumsy sentences. Also think about varying the length of your sentences. Often simple sentences are very effective.
Certainly, try to build your vocabulary; this enables you to write more accurately. Remember, you may also make mistakes with word choice; do not be deterred. Keep trying.
Read a wide variety of texts, particularly editorials and opinion pieces, and think about the author’s turn of phrase and descriptions. Think about the verbs and prepositions authors use.
How do I improve the depth of my ideas?