N 2 S S W T S W
When you are becoming a more skilled writer in the formal style, following the rule that No 2 Sentences Start With the Same Word in a given paragraph means that your writing will immediately improve. It accomplishes two critical goals of effective writing:
1. You immediately vary your sentence structure since each sentence has to begin with a different word. Realistically you might still end up with sentences that all begin with a subject followed by a verb, but the greater likelihood is that you will rearrange parts of the sentences so that a prepositional phrase begins one, rather than the word "the." A bit further, you may begin a sentence with a one word introductory element, such as "However." The result for any one paragraph is likely to be a much more interesting writing style that doesn't distract your reader.
2. You immediately avoid repetitive wording. Repetition is usually a style detriment, when it comes to formal essays. Sentences that all begin with "I" or "The" or "Then" prevent your reader from focusing on your content, and that is not the goal!
Remember that this rule applies by paragraph. You can start fresh every time you have a new paragraph. And like so many rules of this type, the more capable you become, the more you naturally vary your sentence openers and word choices, the freer you will be to actually allow two sentences to start with the same word in a paragraph, just very far apart!