I wrote, "I like to swim in summer." My teacher marked it wrong and said it should read "I like to swim in THE summer." Can anyone offer an explanation, please, why my way is wrong?

I don't know why it was marked wrong. Both with and without "the" are correct. It sounds as if your teacher has heard it and seen it his/her way, so anything else sounds odd.

I feel that way about two other terms:
~~Sounds right: I graduated from high school.
~~Sounds wrong: I graduated high school.
and
~~Sounds right: She is in the hospital.
~~Sounds wrong: She is in hospital.

I know that both versions of both phrasings are in use and considered correct. I just am used to the ones that sound right to me.

You might ask your teacher why that "correction" was made.

??

Thank you for using the Jiskha Homework Help Forum. Although I agree with Writeteacher, perhaps you might apply the following:

1. Mark of a substantive. (noun)

187. (from LousyWriter.com)

The use of "the" before "summer" in the sentence "I like to swim in the summer" is not necessary grammatically. Both "I like to swim in summer" and "I like to swim in the summer" are correct and commonly used.

However, there are cases where the use of "the" before seasons or other time periods can be appropriate. Here are a few examples:

1. When referring to a specific summer:
- "I went on vacation in the summer of 2019."
- "I have fond memories of the summer I spent in Paris."

2. When referring to a specific or known summer that has been previously mentioned:
- "Last year, I went camping in the summer. It was a great experience."
- "The summer before last, we went to the beach every weekend."

3. When referring to summers in general, but emphasizing a specific characteristic or experience associated with them:
- "I love the lazy days of summer."
- "The joy of splashing in the waves is what makes the summer special."

It's possible that your teacher marked it as incorrect due to their preference for using "the" before seasons in general statements. However, both versions are grammatically correct, and the decision to include or exclude "the" can often be a matter of personal preference or style.