Let me know in advance if you're going to be late.
If you're going to be late, send me an email to let me know.
「言ってほしい」には、いろんな表現がありますね。柔らかく言うには、「ほしい」のニュアンスをこめていくつかありますが、「今後は気をつけて」という含みもある状況と判断し、率直に tell me と let me know、（メールでと）具体的に send me an email をあげてみました。
「事前に」は、beforehand や in advance を添えると、釘をさすニュアンスが加味されて、相手は「気をつけよう」という気になると思いますが、特になくても自然に伝わります。
If you will be late, please say so from the next time.
「事前に」というのをきっちりと表現しようとすると
Peekさんのおっしゃるように in advance などといった言い方が必要となり、
ちょっと難しくなりがちです。
ですので、僕はできるだけ中学英語で表現してみました。
If you will be late,
もし遅れるなら
please say so
そう言ってね
from the next time.
次から
いかがでしょうか？
1. If you're going to be late please let me know as soon as you can
2. Please call if you're running late. I need to know!
3. If, one more time, you are late and you don't call me first, you'll be fired!
1. This is a polite request to be informed if the listener will be late.
2. This slightly more urgent request emphasises the importance of calling.
3. This ultimatum amounts to a warning that the listener must call if late or they will be sacked.
Let me know beforehand if you're going to be late.
Let me know ahead of time if you're going to be late.
The word beforehand and the term ahead of time mean in advance. We use both the term ahead of time and the word beforehand a lot in our everyday conversation and in formal settings. These would be great additions to your vocabulary, I hope these two sentences help you out!
この"beforehand"と"ahead of time"とは "in advance"（前もって）という意味になります。
私たちはこの"ahead of time"と "beforehand"をとても一般的に日常会話やフォーマルな場でよく使います。
あなたのボキャブラリーに付け加えると素晴らしいですね。
これらの二つの表現がお役に立てると幸いです！
This way of saying you want someone to tell you they are going to be late, is kind of polite but it also implies that you don't want them to be late or would be annoyed that you didn't tell them because you are asking them to let you know.
Running late can mean you won't be on time but not by a long time.
Sometimes, people know what they would like to do the following day, so they pre-plan. Some people are methodical in their approach to life and will prepare 'to do' lists which are lists of all the things they plan to do on a particular day. Things that will be done the following day for example might include the duration each task will take. So the person will know in advance that he/she will be late for a particular task.
So, you may ask a person who has done his/her time management this way and say:
1. Please tell me if you will be late. The response may be: Yes I will be late because I have to take the children to school before coming here.
2. Please let me know if you will not be early. The response may be: I will definitely not be early due to the traffic congestion in the morning.
ときには、次の日に何をしたいかわかっている人もいて、事前に計画を立てます。几帳面でやりたいことリストを作る人もいます。
次の日にすることは、それがどれくらいの時間がかかるかわかります。そうすることで、事前にあることに遅れると分かります。
1. Please tell me if you will be late.
遅刻するなら教えてください。
Yes I will be late because I have to take the children to school before coming here.
はい、ここに来る前に子供を学校に送っていかなければならないので遅刻します。
2. Please let me know if you will not be early.
早くこられないのなら教えてください。
I will definitely not be early due to the traffic congestion in the morning.
朝の交通渋滞のために確実に早くはないです。
The first example is a question you may ask more informally to confirm that they will arrive at the correct time. The other two are more formal and give the listener a chance to respond once they know more about their arrival time.
If you knew you were going to be late, why didn't tell me?
If you knew you were going to be late, you should have told me beforehand.
事が終わった後で、事前に言ってくれなかった相手に対して、それを非難している状況を想定して文を作ってみました(*^_^*)
If you knew you were going to be late, why didn't tell me?
「遅れるって分かっていたなら、なんで言ってくれなかったの？」
If you knew you were going to be late, you should have told me beforehand.
「遅れるって分かっていたなら、事前に言ってくれればよかったのに」
以上です。
少しでも参考になれば幸いです(*^_^*)
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