明日までに参加するかどうか考えておいてって英語でなんて言うの？

"Please let me know." This expression is casual and polite. Because you are talking about the trip with your friends you can just say "let me know." without having to specify what you what them to tell you. "Please let me know by tomorrow." you can say this when you want them to tell you before the end of tomorrow. "Let me know before tomorrow. " you can say this when you want to know before the end of the day. By not using "please" you add urgency to your request. Meaning it's important to tell you as soon as possible.

Well, there are a few options here. You can ask your friend nicely and mention there is a deadline for their answer. Why there would be a deadline may be because the organisers need to know the final numbers of people attending, for example. Or, if you are simply tired of your friend's procrastination, you could be a little more aggressive and say, "Are you coming or not? I'm tired of waiting for your reply!" Finally, you may take control of this situation and give your friend an ultimatum: "You have until tomorrow to decide. After that, you will not be accepted onto the trip."

▪Tomorrow is the last day that you can decide if you are coming or not. This indicates that his/her cut-off to decide is tomorrow. ▪We need to finalize the arrangements and you only have until tomorrow to decide if you are joining. This is telling the person that he/she is delaying the process to complete the arrangements for the trip and that he/she only have until tomorrow to finally make a decision.

If you are asking your friend if they are coming on a trip then you could simply say 'if your coming as they will know what you are talking about so you don't need to explain. So any of the above sentences would be fine and would tell your friend you need to know by tomorrow if they are coming with you

The two sentences you see provided above are excellent ways to express to someone that you need to know by the next day if they can come with you or not on a trip. In the first sentence you will notice the phrase let someone know. This means to inform someone. This is a phrase that is common in our everyday conversation and it would make a great addition to your vocabulary.

The first way is probably the most polite way to let your friend know that you need to have their answer by tomorrow. The second way is to tell them that they "have to tell you" regarding their decision. The third way is to make it even shorter and to tell them that you must know by tomorrow, and to leave off the phrase, "or not," as it's implied.

When you have a trip planned with a friend but they have not made their decision about going; then you can ask them to make their decision by the next day in the following ways: -Please make your decision by tomorrow -Can you please decide by tomorrow? -I'd appreciate it if you could make your decision by (noon) tomorrow