Excuse me, I will be back, but I must answer the door...I just heard the bell !
We are said to "Answer the door" in English...(an idiom) When we hear a door bell
or a" knock at the door" we are usually obliged to find out who is "at the door"...
We should "excuse ourselves" and inform the teacher ;-D
:Excuse me, I will be back, but I must answer the door !"
英語では"Answer the door"（来客の対応をする）というイディオムを使います。玄関のベルが鳴る又は誰かがドアをノックしたら、通常ドアのところに誰がいるかを確認しなければいけません。
お詫びをして、その旨を先生に伝えるべきです。
例文
:Excuse me, I will be back, but I must answer the door !"
すみません、すぐに戻りますが、お客さんの応対をしなければいけません
Just be polite and courteous and they will be fine. This is if you are just going to leave and come back then you can say, "Please excuse me, someone is at the door," or "I have to get the door but will be right back", or "Please wait while I go get the door".
Now if you are wanting to get away and not come back then you would want to say, "I'm sorry but company have arrived and I need to end class now." or "I'm sorry but I need to reschedule this class since someone has come over now", or "I have to go now, someone is here."
これは丁寧に礼儀正しく言えば、分かってくれます。
もしその場を離れてまた戻ってくるということなら、次のように言えます。
"Please excuse me, someone is at the door"
（すみません、玄関に誰か来ました）
"I have to get the door but will be right back"
（玄関に出てこないといけません、すぐ戻ります）
"Please wait while I go get the door"
（玄関に出てくるので待っていてください）
----
もしその場を離れてもう戻ってこられないのなら、次のように言いましょう。
"I'm sorry but company have arrived and I need to end class now."
（すみません、お客さんが来たので、授業を終わりにしないといけません）
"I'm sorry but I need to reschedule this class since someone has come over now"
（すみません、お客さんが来たので授業は別の日にしないといけません）
"I have to go now, someone is here."
（お客さんが来たので、もう行かないといけません）
This is a fairly simple situation since most people understand whenever someone answers the door. But these are common ways of telling someone that you need to answer the door. Also saying that it may be important is a very polite way of saying this.
これはかなりシンプルな状況ですね、家に人が訪ねてくることがあることはみんな知っていますから。
上記は、来客の対応をしなければならないことを伝える一般的な言い方です。また、"it may be important"（大切なことかもしれない）は、これについて伝えるとても丁寧な言い方です。
The two sentences you see provided above are terrific ways to express to your listener that there is someone at your door and you will be right back. In the first sentence you will see the word sec. This is an abbreviation of the word second, but we use this to describe a small amount of time. This word is appropriate for informal settings.
I'm sorry I'll be right back someone is at the door!
Someone is at the door means someone is ringing the bell and you need to answer the door
A polite way of excusing yourself is to say 'please excuse me' or 'I'm sorry'
I'll be right back suggest you will not be long just a few minutes and then return
Someone is at the door' は「誰かがドアベルを鳴らしていて、出ないといけない」という意味です。
'please excuse me' または 'I'm sorry' は、席を外すときの丁寧な表現です。
'I'll be right back'（すぐに戻ります）は「数分で戻る」というニュアンスです。
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