1. If I didn't wear a school uniform, then I would be happy every day.
2. If I didn't wear a school uniform, I would be happy every day.
(Which one is correct? Are both OK? Which one is popular? What is the difference between the two sentences because of 'then'? Thank you.)
Both are OK and both are commonly used. Even if the word "then" is not there, it's implied -- because of "if."
Both sentences are grammatically correct, but they have slightly different implications.
The first sentence, "If I didn't wear a school uniform, then I would be happy every day," suggests a cause-and-effect relationship between not wearing a school uniform and being happy every day. The word "then" in this sentence is used to indicate a logical consequence or outcome. It implies that not wearing a school uniform directly leads to being happy every day.
The second sentence, "If I didn't wear a school uniform, I would be happy every day," suggests a hypothetical scenario or condition without explicitly stating a cause-and-effect relationship. In this sentence, there is no need for the word "then" because the speaker is simply expressing a hypothetical situation.
Both sentence structures are commonly used, but the second sentence, without the word "then," is more commonly used in everyday speech. The inclusion of "then" adds emphasis to the cause-and-effect relationship and may be more commonly used in formal writing or when emphasizing a logical consequence.