1) "You need to respond to at least 100 messages a day." 2) "You need to respond to at least 100 messages per day." 3) "You need to respond to at least 100 messages daily." Which is the most appropriate and mostly used in formal letters?

A in that sense is a preposition meaning the same thing as per. Any dictionary will have that as a definition for a (Dictionary.com): preposition each; every; per: ten cents a sheet; three times a day. Oxford Dictionary Online and Merriam-Webster have similar entries. So there is no difference in meaning between your examples.

2021年2月7日 · @Boyep The meaning of the first sentence is clear: it happened each / every day before this. What's left out is the number of times it happened. For instance, it happened twice per day before this or it happened four times per day before this. –

2022年10月9日 · Therefore, "per day" means "for each day". In your example of "he run 20 km per day during last 10 days", it means "he run 20 km for each day during last 10 days". As you suggested, it is not average, it refers to a day. Another example would be "the doctor asked me to take 2 pills per day" = "the doctor asked me to take 2 pills for each day".

2019年4月6日 · "per" generally means an averaging. One may study for 5 hours one week, and 15 the next, and achieve 10 hours per week. "each" generally means either an equal distribution, or a minimum, or both. Studying 10 hours each week would mean at least 10 hours every week.

There are many good techniques that can help, for example you could learn only a few words per day, and repeat them over time. Flash cards are a practical application of this. If over time you feel you forget the words you knew only a few days back, then you are learning them too fast, and you should slow down.

2020年3月6日 · What is the difference between "each day" and "every day"? For example: We talk every day. We talk each day.

2019年11月26日 · Saying that somebody has to pay a certain amount per month (or per hour, per day, per week, etc.) does not imply anything as to when and how frequently the payments are to be made. The only information that such a formulation conveys is how the total amount of the payment, whenever it is made, is related to the period of time for which it is ...

2020年11月5日 · "Once every day" has the emphasis on it being extremely important that you don't miss a day. Such as a doctor telling you to take your medicine once every day. This doesn't really apply here. "Once per day" is similar to "once a day" but a bit more formal. Probably more formal than is necessary here but it wouldn't be weird.

2019年9月17日 · Every second day means a 2 day cycle, which is every other day. So 2c would be correct. (and that other thread would be correct too) Attempting to be clear: Every day with a visit = 1 day cycle = every single day = every day. One day empty, one day with a visit = 2 day cycle = every second day = once every two days = every other day