Learn Hardcore Japanese: senpai ha setumei ga zyouzu da. sore ni, koe mo kikiyasui. - My senior explains well. In addition, her voice is easy to hear.

X は sets the topic (who we’re talking about): 先輩は “as for the senpai,”
Y が marks the thing X is skilled at/likes: 説明が上手 “is good at explaining.” This pattern is used with words like 上手, 下手, 好き, 嫌い, 得意, 苦手.

Could I say 説明は上手だ instead?

It’s possible but shifts the focus:

先輩は説明が上手だ = As for the senpai, (they) are good at explaining. (ability of the person)
先輩の説明は上手だ = The senpai’s explanations are good. (quality of the explanations)
説明は上手だ alone sounds incomplete because it lacks who is good at explanations.

Why is 上手 followed by だ, but 聞きやすい doesn’t take だ?

上手 is a na-adjective, so it needs a copula: 上手だ / 上手です.
聞きやすい is an i-adjective formed with the suffix 〜やすい, so it doesn’t use だ: 聞きやすい / 聞きやすいです.

What exactly does それに mean here? How is it different from そして, それから, しかも, さらに?

それに: “in addition; besides” (adds another point; conversational and natural here).
そして: simple “and then/and,” neutral linking; often narrative sequencing.
それから: “after that/and then,” temporal sequence.
しかも: “moreover/what’s more,” emphasizes an unexpected or impressive add-on.
さらに: “furthermore,” a bit more formal; often escalates the degree.

What is も doing in 声も聞きやすい?

も means “also/too.” It replaces the usual case particle (が here). So 声も聞きやすい = “(Their) voice is also easy to hear,” adding to the first point (good at explaining).

Should it be 声が聞きやすい instead of 声も聞きやすい?

Both are correct. Use:

声が聞きやすい when it’s a standalone statement.
声も聞きやすい when you’re adding it to a previous positive trait (as in this sentence).

Does 聞きやすい mean easy to physically hear or easy to understand?

Usually “easy to hear” (volume/clarity/less background noise). For “easy to understand by listening,” 聞き取りやすい is more precise. Alternatives:

声がはっきりしている (clear)
声が通る (projecting, carries well)

Is 上手 the same as うまい? What about 得意?

上手（じょうず）: “skillful,” polite/neutral, often used to praise others.
うまい: casual; can sound blunt/masculine; also means “tasty.”
得意: “to be good at / one’s strong suit,” commonly for oneself. Tip: Avoid overusing 上手 about yourself; 得意 is safer.

Why is 説明 a noun here? Could I say 説明するのが上手だ?

Yes. 説明 is a verbal noun (“explanation”/“explaining”). Both are natural:

説明が上手だ (set pattern)
説明するのが上手だ (explicitly verbal; slightly more action-focused) Using の to nominalize (説明するの) is very common in speech.

How can I make the sentence more polite?

Use です/ます:

先輩は説明が上手です。それに、声も聞きやすいです。 Or connect them:
先輩は説明が上手で、声も聞きやすいです。

Can I connect without それに? What about 〜で or 〜だし?

Yes:

先輩は説明が上手で、声も聞きやすい。 (neutral connection)
先輩は説明が上手だし、声も聞きやすい。 (adds a “plus also/and on top of that” nuance)

Do I need to say 先輩の声 to be clear?

Not necessary; Japanese often omits obvious possessives. 声も聞きやすい is understood as “(the senpai’s) voice.” If clarity is needed, say 先輩の声も聞きやすい.

Is 先輩 used like a title? Do I add さん?

You can address someone as 先輩 directly (common at school/clubs) or attach it to a surname: 佐藤先輩.
Adding さん after 先輩 (先輩さん) is unusual. Use 先輩 alone or name + 先輩.

How do you read each word? Any pitfalls?

先輩: せんぱい
説明: せつめい
上手: じょうず (watch out: 上手 can also be うわて in other contexts)
声: こえ
聞きやすい: ききやすい
The whole thing: せんぱいはせつめいがじょうずだ。それに、こえもききやすい。

Could I say 上手な説明だ?

That means “It’s a skillful explanation” (evaluates that particular explanation), not “(He/She) is good at explaining.” For ability, stick to 説明が上手だ or 説明するのが上手だ.

What are good synonyms to describe the voice?

声がはっきりしている (clear)
声が通る (projects/carries)
滑舌がいい (good articulation)
聞き取りやすい (easy to catch/understand by ear)

Is それに too casual for writing?

It’s fine in general writing and conversation. In more formal writing, さらに, 加えて, or しかも may fit better depending on nuance.

Does ending with だ sound masculine?

だ is plain style, not inherently masculine. It’s neutral in statements. Use です for polite tone.

What’s the difference between 先輩も説明が上手だ and 声も聞きやすい?

先輩も説明が上手だ: “The senpai is also good at explaining” (compares the senpai to someone else previously mentioned).
声も聞きやすい: “(Their) voice is also easy to hear” (adds another trait of the same senpai).

Why not 聞こえやすい?

聞こえる = “to be audible.” 聞こえやすい is used for sounds that tend to be audible (e.g., high pitches), but for evaluating a person’s speech, 聞きやすい or 聞き取りやすい is more natural.

How do I negate or change tense?

上手だ → negative: 上手ではない / 上手じゃない; past: 上手だった
聞きやすい → negative: 聞きやすくない; past: 聞きやすかった Polite forms: 上手ではありません, 聞きやすくありません.

Any note about spacing/punctuation?

Standard Japanese doesn’t use spaces between words. The natural punctuation is: 先輩は説明が上手だ。それに、声も聞きやすい。