Japanese people used to be shorter (in the past), so the ceilings of old houses are lower.
「昔の日本人は今よりも背が低かったので昔の家は天井が低い」というのはこのようになります：
Japanese people used to be shorter (in the past), so the ceilings of old houses are lower.
この場合は 「以前~、今は~」というused to~のパターンを使って、「日本人は以前に背が低かったです」と言い回しです。
in the pastは「過去に」という意味で、「昔」の言い換えとなります。
「昔の家」はシンプルにold houseにしました。
「天井が低い」の「低い」というのは、shortではなくlowとなります。
また、「古民家」というのはold folk houseとなり、
「私は古民家に住んでいる」はI live in an old folk house.といいます。
As the average height of Japanese people were shorter in the past, the ceiling is naturally designed lower in a house built in those days.
「日本人は背が低かった」は簡単に言うと"Japaense people were shorter"でもいいですが、ここでは少し客観的に（ちょっと大人な言い方で）、「日本人の平均身長は・・・」と表現してみました。"shorter in the past"だけで、「今よりも」のニュアンスは含まれます。
また、「天井が低い」もシンプルに"the ceiling is lower"でももちろんわかりますが、"is designed low"をすることで、「低く【作られている】」というニュアンスを入れてみました。
日本の伝統文化や古民家のことなど、ぜひいろんな先生に教えてあげてください！私も自分のクラスの生徒には、世に知られていない日本文化をどんどん発信するよう、促しているところです。「そういったちょっと難しいことでも頑張って言ってみよう！」と思う気持ちを、ぜひ応援させてください。うまく伝わることを願っています！
Old Japanese buildings have low ceilings on account of people being shorter in olden times
if you wish to describe that one thing is due to another thing, or one thing occurred because of another thing, you may use the form 'on account of'. For example, the British pound is low compared to other currencies at the moment on account of the uncertainty about our economic future outside the EU.
あることが別のことのせいでそうなったと言いたいときは、'on account of'(～のために、～の理由で）というフレーズを使うといいでしょう。
例：
The British pound is low compared to other currencies at the moment on account of the uncertainty about our economic future outside the EU.
イギリスのポンドは、EU外の今後の経済の不安定さの理由で、現時点では他の通貨に比べると価値が低い。
The ceilings for old houses are low because they Japanese people at that time were short.
*The ceilings for old houses are low because Japanese people at that time were short.
This means that the ceilings were designed in a way to cater for the short people at that time. However if you are comparing remember we say
short: for one thing
shorter: two things
shortest: more than two things
*The ceilings for old houses are low because Japanese people at that time were short.
これは、天井が当時の背の低い人たちに対応するように設計されていたという意味です。ただし、比較するときは以下のように表現することを覚えておきましょう。
short: 1つの物事について
shorter: 2つの物事
shortest: 3つ以上の物事
The ceilings in these older builds are lower because the average Japanese person was shorter back then.
an older build - this is one way to say "a building that was build in an earlier time" or "an older building'
back then / back in the days - this means "in the past"
an older build
「以前に建てられた建物」や「古い建物」といった言い方の一つです。
back then / back in the days
”過去に "という意味です。
The ceilings in older houses are lower as the Japanese people were shorter in the past
In the past the Japanese were shorter that's why the ceilings are lower in older houses
To say 'The ceilings in older houses are lower as the Japanese people were shorter in the past' this explains that it's in older house and not newer ones and in the 'past' means before now
The ceilings in older houses are lower as the Japanese people were shorter in the past'
（昔の日本人は今より背が低かったので、古い家の天井は低い）
- それが、新しい家のことではなく、古い家のことだと説明しています。'in the past'は「昔は」という意味です。
In the past, Japanese people were shorter than they are now, so older houses have lower ceilings.
Older homes have lower ceilings because Japanese people were shorter in the past.
Long ago, homes were built with lower ceilings because the Japanese were shorter back then.
We use phrases such as, "in the past" and, "long ago" to describe time periods in history. We may or may not have been alive ourselves during these times.
A simple way to say this is also to begin the sentence by describing the homes as, "older" and ending the sentence with, "in the past."
過去の時代を表すときには、「in the past（昔は）」 「long ago（ずっと以前）」などが使われます。その時代に私たちは生きていたかもしれませんし、生きていなかったかもしれません。
これは、文を「older homes（昔の家）」で始めて、「in the past」で終えると、シンプルに表せます。
Ceilings of older houses in Japan are lower because Japanese people were shorter in the past
The old houses in Japan have lower ceilings because people in the past were much shorter than they are now
When you want tot explain to someone that long ago Japanese people were shorter than they are now, and that is the reason why ceilings of older houses are low; you may explain this in the following ways:
-Ceilings of older houses in Japan are lower because Japanese people were shorter in the past
-The old houses in Japan have lower ceilings because people in the past were much shorter than they are now
古民家の天井が低いのは昔の日本人は今よりも背が低かったからだと伝える言い方です：
【例文】
-Ceilings of older houses in Japan are lower because Japanese people were shorter in the past
［訳］昔の日本人は背が低かったので、古い家の天井は低いです
-The old houses in Japan have lower ceilings because people in the past were much shorter than they are now
［訳］昔の日本人は今よりも背が低かったので、古い家の天井は低いです
Old Japanese houses have low ceilings because people used to be shorter.
おっしゃられている内容は以下のようにも表現できると思いました(*^_^*)
Old Japanese houses have low ceilings because people used to be shorter.
「昔人々は背が低かったので、古い日本の家は低い天井を持っています」
ceiling「天井」
used to ～「かつては～だった」
以上です。
少しでも参考になれば幸いです(*^_^*)
★★大阪カフェレッスン英会話講師KOGACHI
アメリカ合衆国 
日本 
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