In the following sentence improvement question, the underlined part is since he has known them, began to quickly swim: One night they sank through the shiny water, and for the first time since he has known them, began to quickly swim. Choose the best improvement.

Introduction / Context:
This sentence improvement question examines two aspects of good English usage at the same time: correct tense choice with since and natural adverb placement. The sentence reads One night they sank through the shiny water, and for the first time since he has known them, began to quickly swim. The underlined part since he has known them, began to quickly swim is slightly awkward and contains a tense mismatch. The clause describes a past event, and since is used to refer to a period extending up to that past point, so the verb form must match a past time frame, not present perfect.

Given Data / Assumptions:

• Sentence: One night they sank through the shiny water, and for the first time since he has known them, began to quickly swim.
• Underlined segment: since he has known them, began to quickly swim.
• Options involve different tense forms and word orders.
• Context: The narrator is describing a particular night in the past when something new happened after a long acquaintance.

Concept / Approach:
When we use since to refer to a period leading up to a past time point, the correct tense in narrative writing is usually past perfect: since he had known them. This tells us that his knowing them started earlier and continued up to that night. The phrase began to swim quickly is also more natural than began to quickly swim, because splitting the infinitive in this context is less idiomatic in formal writing. Therefore, the best improvement is he had known them, began to swim quickly, which corrects the tense and smooths the word order.

Step-by-Step Solution:

Verification / Alternative check:
Re-read the improved sentence: One night they sank through the shiny water, and for the first time since he had known them, began to swim quickly. Now the tense sequence is logical: the state of knowing them (past perfect) extends up to that particular night, and the new action began to swim quickly is in simple past, matching sank. This creates a smooth, coherent narrative. The present perfect form has known would typically be used if we were speaking from the present about an ongoing relationship (since he has known them, they have always...), which is not the case here.

Why Other Options Are Wrong:
Option B (he have known them, begin to quickly swim) is clearly ungrammatical: have should be has or had, and begin does not fit the past narrative. Option C (he knew them, began to swim quickly) uses simple past knew, which is possible but weaker than past perfect in marking the prior state; exam patterns usually prefer past perfect after since in such storytelling contexts. Option D (No improvement) is incorrect because has known conflicts with the past narrative and to quickly swim is less natural. Option E (he had known them, begun to quickly swim) wrongly changes began to begun, which breaks the simple past structure, and also retains the less preferred to quickly swim order.

Common Pitfalls:
Many learners misuse present perfect (has known) when the viewpoint is clearly in the past. Another common issue is ignoring subtle improvements in word order, such as placing adverbs after the verb phrase rather than splitting the infinitive. While split infinitives are not always wrong in modern English, exam standards still tend to favour more traditional patterns like swim quickly. Paying careful attention to the narrative time frame and to smooth, conventional phrasing will help you choose the best improvement in such questions.

Final Answer:
The correct improvement is he had known them, began to swim quickly., giving: One night they sank through the shiny water, and for the first time since he had known them, began to swim quickly.