Japanese does not have future tense modal verbs like “will” or “be going to” in English. To show that we are talking about the future, we need to use words such as 明日(tomorrow) and 来年(next year). The basic form of all Japanese verbs is by default present tense and future tense at the same time. We must read a sentence in context in order to know the tense.
The problem is that in beginner’s textbooks, usually the following examples are given to learners:
昨日はいい天気でした。(Yesterday, the weather was good.)
今日はいい天気です。(Today, the weather is good.)
明日はいい天気でしょう。(Tomorrow, the weather will be good.)
If the teachers do not explain in details, then it is very likely that learners will think that でした=”was”, です=”is”, でしょう=”will be”. They may then translate all the “will” sentences in English into “でしょう” sentences in Japanese, thinking that でしょう indicates future tense in Japanese, which is, unfortunately, a very serious mistake. (For your information, でしょう is in the polite formal form. Its informal form counterpart is だろう. In this article, we will useでしょう only.)
So what is でしょう for? Is “明日はいい天気でしょう” a grammatically correct sentence?
でしょう does not indicate any tenses. Instead, it shows that the statement before it is a guess or speculation. We use でしょう only when we are not 100% sure of what we are saying. Because we are not god and thus can’t be sure that the weather will be good tomorrow, 明日はいい天気でしょう is grammatically and logically correct. However, if we say明日は日曜日でしょう (Tomorrow will be Sunday), it sounds very unnatural, because “tomorrow is Sunday” is a fact, not a guess.
Becauseでしょう indicates guess, it can be used for current events and past events that we are not certain. For example, you think that Mr Tanaka goes to his office by bus every day but are not 100% sure, you can say 田中さんは毎日バスで会社へ行くでしょう (current event); or you think that many people may have died in yesterday’s earthquake but are not sure, you can say 多くの人が死んだでしょう (past event). As you can see, でしょう really has nothing to do with tenses. And after reading the newspaper and learn that 2000 people died in the quake, you cannot use でしょう again because you are certain now.
And since でしょう can only be used to indicate guess, we cannot use it when we are talking about what we are going to do. For example, 私は四時にくるでしょう (I will come at 4 o’clock) is incorrect because I should know what I will do and don’t’ need to make guesses. “私は四時にくる” is enough to indicate our intention.