Learn Hardcore Japanese: sono bukatu no senpai ha kibisikatta ga, ima omoeba anitati ha yoku rensyuusaserarete tuyoku natta to omou. - The seniors in that club were strict, but looking back, I think my brother and his teammates were made to practice a lot and became strong.

部活: strongly associated with Japanese school clubs (sports teams, cultural clubs) as part of school life.
クラブ: more general “club”; can be school-related, but also things like sports clubs outside school, hobby clubs, even nightclubs depending on context.

So その部活の先輩 is specifically talking about seniors in a school club context, not just any club.

Why is 先輩 used without たち even though it seems to be talking about more than one person?

Japanese often leaves the distinction between singular and plural to context.

先輩 can mean senior / upperclassman as an individual.
But 先輩 without たち can also refer to the seniors as a group.

In this sentence:

その部活の先輩は厳しかったが…

the context (club seniors being strict) makes it natural to understand it as plural: the seniors in that club were strict. Adding たち (先輩たち) would also be correct but is not necessary.

What nuance does 厳しかったが have here? Is が just “but”? Could I use けど instead?

Yes, が here works like “but / although”.

その部活の先輩は厳しかったが、…
The seniors in that club were strict, but …

厳しかった: past tense of 厳しい (strict, severe).
が at the end of a clause is a conjunctive particle meaning “but / although”.

You could say:

厳しかったけど、…
厳しかったけれど（も）、…

These are more conversational. Sentence-final が in writing often feels a bit more formal or “essay-like” than けど.

What does 今思えば mean exactly, and why is ば used?

今思えば literally breaks down as:

今 – now
思う – to think
思えば – “if (I) think (about it)”

So 今思えば means something like:

“If I think about it now,”
“Looking back now,”
“Now that I think about it,”

The 〜ば form here is a conditional, but it’s used in a set phrase that introduces reflection from the current point of view.

So the flow is:

今思えば、兄たちはよく練習させられて強くなったと思う。
When I think about it now, I think my older brothers were made to practice a lot and became strong.

Why is 思う used twice (in 今思えば and 〜と思う)? Is that redundant?

It looks repetitive in English, but in Japanese it’s natural and the two 思う have different roles:

今思えば – a fixed expression meaning “When I think about it now / Looking back now”
This sets the perspective of the speaker.
〜強くなったと思う – と思う here is “I think (that) …”
This gives the speaker’s opinion or conclusion.

So the structure is like:

When I think about it now, I think that X.

English also sometimes has this double-think structure:
“Now that I think about it, I think they really improved.”

What does 兄たち mean here? Why is たち added to 兄?

兄（あに） by itself usually means my older brother (singular).
兄たち is the plural form: my older brothers (or the older brothers in my family / group).

So:

兄たちはよく練習させられて…

means my older brothers were often made to practice… (more than one brother).

〜たち is a common plural marker for people:

子ども → 子どもたち (children)
先生 → 先生たち (teachers)
友だち → 友だちたち (friends – although often just 友だち is used)

What is the nuance of よく in よく練習させられて? Does it mean “well” or “often”?

In this context, よく means “a lot / often / frequently”, not “well”.

よく練習する – practice a lot, practice often
よく勉強する – study a lot

So 兄たちはよく練習させられて means:

“My older brothers were made to practice a lot.”
“My older brothers were often forced to practice.”

If you wanted to say “practice well (skillfully)”, you’d use something like:

上手に練習する
うまく練習する

How does 練習させられて work grammatically? What is this form and what nuance does it have?

練習させられて is the causative passive of 練習する (to practice), in て-form.

Step-by-step:

Base verb: 練習する – to practice
Causative: 練習させる – to make/let someone practice
Causative passive: 練習させられる – to be made to practice (often implies against one’s will)

Then it’s put into て-form:

練習させられて – “(they) were made to practice and … / being made to practice, (they) …”

Nuance:

It strongly suggests the brothers did not decide on their own; someone (the strict seniors, presumably) made them practice.
It can have a slightly “forced / tough” feeling: they had to go through hard training.

So:

兄たちはよく練習させられて強くなった

≈ My older brothers were often made to practice, and (as a result) they became strong.

What role does the て in 練習させられて強くなった play? Is it “and” or “because”?

The て form here connects two actions/states:

練習させられて – were made to practice
強くなった – became strong

The connection can be understood as:

sequential: were made to practice and (then) became strong, or
causal: because they were made to practice, they became strong

Japanese 〜て is flexible and often has both “and/then” and “so/because” flavors depending on context. Here, in natural English, it would most likely be translated with a causal nuance:

They were made to practice a lot, so they became strong.

Why is it 強くなった and not 強いになった? How does なる work with adjectives?

With い-adjectives, you don’t add に; instead, you change the adjective into its adverbial form (〜く) before なる:

強い → 強くなる – to become strong
暑い → 暑くなる – to become hot
新しい → 新しくなる – to become new

So the past tense is:

強くなった – became strong

For な-adjectives and nouns, you do use に:

静か（な） → 静かになる – to become quiet
きれい（な） → きれいになる – to become pretty
先生 → 先生になる – to become a teacher

So 強いになった is ungrammatical; it must be 強くなった.

What does the と in 強くなったと思う do? Why is と needed before 思う?

The と before 思う is the quotative particle. It marks the content of what is being thought, said, felt, etc.

Structure:

[clause] と思う – I think (that) [clause].

So:

強くなったと思う

literally: I think, “(they) became strong.”
Natural English: I think they became strong.

This と is used with many verbs of saying/thinking:

〜と言う – say that 〜
〜と聞く – hear that 〜
〜と感じる – feel that 〜
〜と思う – think that 〜

Without と, the sentence would be ungrammatical here.