Learn Hardcore Japanese: anitati ha syakaizin ni natte kara totemo isogasiku narimasita. - After my older brothers became working adults, they became very busy.

兄 means “older brother” (my own), but it’s usually singular.
たち is a plural-like suffix. 兄たち means “(my) older brothers.”
You wouldn’t normally add お (the honorific) to your own brother in neutral narration, so お兄さんたち here would sound odd or too childish/polite toward your own family.

So 兄たち = my older brothers (more than one). If it were just one, you’d say 兄は.

What is the role of は in 兄たちは? Could I use が instead?

は marks the topic: what you’re talking about, the “as for X” part.

兄たちは社会人になってから…
- “As for my older brothers, after they became working adults…”

Using が here would be unusual, because this sentence is just giving information about an already-known topic (my brothers), not contrasting them or presenting them as brand‑new information.

兄たちが社会人になってから…
Could be used in some contexts (for example, inside a larger sentence), but by itself it tends to sound like you’re emphasizing they (and not someone else) became working adults and then got busy. For a neutral statement, は is the natural choice.

What exactly does 社会人 mean? Is it just “company employee”?

社会人 (しゃかいじん) literally means “member of society,” but in everyday Japanese it means:

an adult who has entered the working world (not a student anymore)

Key points:

It doesn’t have to be a company office worker; it can be any working adult (freelancer, shop worker, civil servant, etc.).
It contrasts mainly with 学生 (student).
会社員 specifically means “company employee.”
A 社会人 can be a 会社員, but also other kinds of workers.

So the sentence is about after my older brothers started their working‑adult life (after graduating and entering the workforce), not just “after they joined a company” in a narrow sense.

Why is there に after 社会人 in 社会人になって?

With the verb なる (“to become”), に marks what you turn into:

X になる = “become X”
学生になる – become a student
有名になる – become famous
社会人になる – become a working adult

So 社会人に is “into a 社会人,” and 社会人になって is “becoming a 社会人 and then…” / “after becoming a 社会人…”

What does 〜てから mean in 社会人になってから?

〜てから means:

after doing X / since doing X

Structure:

Verb in て‑form + から
社会人になってから
= “after becoming a working adult” / “since they became working adults”

Nuances:

It shows sequence: first A happens, then B.
Often implies an ongoing state from that time onward, similar to English “ever since ~”:
- 社会人になってから、ずっと忙しいです。
  “I’ve been busy ever since I became a working adult.”

In your sentence, it sets the time frame: after they became working adults, they became very busy.

Why is 忙しい changed to 忙しく in とても忙しくなりました?

忙しい is an い‑adjective meaning “busy.”

When an い‑adjective comes before なる (“to become”), you change 〜い to 〜く:

忙しい → 忙しく
大きい → 大きく
寒い → 寒く

Pattern:

A‑い + く + なる = “to become A”
- 忙しくなる – to become busy
- 大きくなる – to become big
- 寒くなる – to become cold

So:

忙しくなりました = “(they) became busy”
とても忙しくなりました = “(they) became very busy”
(とても modifies 忙しく)

Could I say とても忙しいになりました instead?

No. That’s ungrammatical.

Reasons:

With なる, い‑adjectives must take the 〜く form:
- 忙しい → 忙しくなる (NOT 忙しいになる)
The adverb とても goes in front of the adjective stem:
- とても忙しくなりました

Correct forms:

忙しくなりました – became busy
とても忙しくなりました – became very busy

So とても忙しいになりました is wrong; it must be とても忙しくなりました.

Why is it なりました and not just なった?

Both come from なる (“to become”), but:

なりました = polite past
なった = plain past

So the choice is about politeness level, not meaning:

兄たちは社会人になってからとても忙しくなりました。
Polite; suitable for talking to someone you’re not close to, or in writing/essays.
兄たちは社会人になってからとても忙しくなった。
Casual; used with friends/family, in diaries, etc.

The underlying meaning “became very busy” is the same.

In English I might say “have been very busy since becoming working adults.” Why does Japanese use a simple past なりました?

Japanese often uses past tense to describe a change of state:

忙しくなりました – “(they) became busy” (focus on the change)

If the context is clear, Japanese doesn’t always explicitly say “and they are still busy now.” It’s understood from the situation.

If you really wanted to stress an ongoing busy state since then, you might say:

社会人になってから、ずっととても忙しいです。
“They have been very busy ever since they became working adults.”

But for a simple narrative about what happened after they became 社会人,
忙しくなりました is natural and enough.

Can I change the word order, like putting 社会人になってから at the beginning?

Yes. Japanese word order is flexible as long as the particles are correct.

Your sentence:

兄たちは社会人になってからとても忙しくなりました。

You can also say:

社会人になってから、兄たちはとても忙しくなりました。

Both are natural. The difference is focus/order of information:

Starting with 兄たちは:
“As for my brothers, after they became working adults, they became very busy.”
Starting with 社会人になってから:
“After they became working adults, my brothers became very busy.”

Meaning is the same; it’s mostly about what you want to emphasize first.

Where should とても go? Could I say 忙しくとてもなりました?

とても is an adverb that normally comes right before the adjective or adjective‑like part it modifies.

Correct:

とても 忙しく なりました。
“They became very busy.”

Incorrect:

忙しくとてもなりました ❌
This is ungrammatical and not natural.

Some acceptable variations:

兄たちはとても忙しくなりました。
社会人になってから、兄たちはとても忙しくなりました。

So keep とても immediately before 忙しく.