So vs Therefore: Informal vs Formal

So and therefore both introduce results, but they belong to different levels of formality. So is informal and very common in spoken English — it joins two clauses with a comma: "I was tired, so I went to bed early." Therefore is formal and used mainly in written English — it usually starts a new sentence or follows a semicolon: "I was extremely tired. Therefore, I went to bed early." Choosing between them depends on how formal your writing needs to be.

In everyday conversation, so is perfectly natural and nobody will judge you for using it. In essays, reports, and academic writing, therefore sounds more professional and precise. Therefore can also appear in mid-sentence position after the subject: "I was therefore unable to attend the meeting." This mid-position use is particularly common in formal and academic English. A common mistake is using "so" to start a sentence in formal writing — while it is becoming more accepted, "therefore" or "consequently" remain the better choices for essays and exams. Learning to switch between informal and formal result connectors is an important skill for B1-B2 learners.