Learn Hardcore Japanese: ziyuuseki ha benri da ga, hobo itumo ippai ni naru. - Non-reserved seats are convenient, but they’re almost always full.

It’s the unreserved seating section, read as jiyūseki. On many JR trains (including Shinkansen), some cars are designated as unreserved: you don’t pick a specific seat in advance; you board and take any open seat. This contrasts with 指定席 (reserved seats), where a specific seat is assigned.

How do you pronounce the whole sentence?

Kana: じゆうせきはべんりだが、ほぼいつもいっぱいになる。
Romaji: Jiyūseki wa benri da ga, hobo itsumo ippai ni naru.

Why is は used after 自由席 instead of が?

は marks the topic, framing what you’re talking about before giving a comment about it. Here, the speaker is making a general statement about unreserved seats. Using が would mark 自由席 as the grammatical subject and add a stronger focus/contrast, but は is the default, natural choice for broad, descriptive comments.

What is 便利, and why is だ used?

便利 is a na-adjective meaning “convenient.” Na-adjectives need だ (plain) or です (polite) when they function as predicates. So 便利だ is “is convenient” in plain style.

What is が doing here—subject marker or “but”?

Here が is the conjunction meaning “but/although,” attached to the predicate 便利だ to form 便利だが. It’s not the subject marker in this sentence.

Is だが too formal? How would I say this politely or casually?

だが is somewhat formal/written. Alternatives:

Polite: 自由席は便利ですが、ほとんどいつも混んでいます。
Casual: 自由席は便利だけど、ほとんどいつもいっぱいになる。
Neutral/plain: 自由席は便利だが、ほとんどいつも満員になる。

Why use になる instead of だ/です?

〜になる means “to become,” emphasizing the change that happens each time (e.g., each trip, it ends up full).

いつもいっぱいだ = is always full (state).
ほぼいつもいっぱいになる = almost every time, it becomes full (process/result).

Why is に used with なる here?

With nouns and na‑adjectives, the resulting state before なる takes に. Patterns:

Noun/na‑adj + に
- なる: 静かになる, きれいになる, いっぱいになる.
i‑adj + く
- なる: 寒くなる, 安くなる.

What does ほぼ mean, and how is it different from ほとんど/たいてい/だいたい?

ほぼ: nearly/almost (close to 100% or complete), often with quantities/frequency, e.g., ほぼ毎日.
ほとんど: almost/mostly; very common with frequencies and amounts, e.g., ほとんどいつも.
たいてい: usually/generally (not near-100%); good for habits.
だいたい: roughly/approximately; about, in general.

Is ほぼいつも natural? What are alternatives?

ほぼいつも is understandable and used, but many speakers prefer ほとんどいつも for “almost always.” Other options:

たいてい (usually)
ほぼ毎回 (almost every time; slightly formal/written)
ほぼ常に (nearly always; formal)

What exactly does いっぱい mean here, and how is it different from 混んでいる, 満席, or 満員?

いっぱい: “full.” As a predicative state (いっぱいだ) or result (いっぱいになる).
混んでいる: “crowded,” not necessarily completely full.
満席: “all seats taken” (seating context: theaters, planes; also cars with only seats).
満員: “at capacity with people” (standing space maxed out; trains/buses).
For unreserved train cars:
If you mean no seats left, 満席 fits that idea but is more natural when explicitly about “seats.”
If you mean the car is packed, 満員 or とても混んでいる is better.
いっぱいになる is a plain, all‑purpose way to say it ends up full.

Is the spacing and comma in the example how Japanese is usually written?

Normally, Japanese isn’t written with spaces between words. A natural, no‑space version is:
自由席は便利だが、ほとんどいつもいっぱいになる。
The comma is optional and marks a pause; many writers place one before the contrasting clause.

Does 自由席 refer to the seats or the cars—what becomes full?

In everyday speech 自由席 can stand for “the unreserved seating section/cars.” So saying 自由席は…いっぱいになる is understood as “the unreserved section (cars) becomes full.” If you need to be precise, you can say 自由席の車両は満員になる (the unreserved cars get packed) or 自由席の座席は満席になる (all unreserved seats get taken).