If you find writing topic sentences tricky, it might be helpful to prepare a bank of topic sentence stems or starters. For example, the first two topic sentences in a basic critical essay on the ...

Statements are the most common type of sentence. They tell the reader a fact or idea about a single topic. They must always end in punctuation, usually a full stop. Statements are the most common ...

Writing a strong topic sentence that establishes the subject of the paragraph Using "echo words" -- i.e., consistent terms for the concepts throughout Applying transitions to link sentences logically ...

Express one idea per sentence. Use your current topic — that is, what you are writing about — as the grammatical subject of your sentence (see Verbs: Choosing between active and passive voice).