Is this sentence proper?
They had swum across the lake several times.
Yes, it's perfect.
Yes, the sentence "They had swum across the lake several times" is grammatically correct. It is written in the past perfect tense, using the verb "swum" to indicate an action that had taken place in the past. The subject "they" refers to a group of people, and the phrase "across the lake" provides additional information about where the swimming took place. Overall, the sentence is properly constructed and conveys its intended meaning.
To determine if a sentence is grammatically correct, you can check for subject-verb agreement, verb tense consistency, and proper word usage. In this case, the verb "swum" agrees with the subject "they," and the past perfect tense is used correctly. Additionally, the sentence is structured properly, with no glaring errors in word choice or syntax.