Rain in the city

Question

東京は今雨が降っている。

literally: Tokyo now rain is falling.

Question: why Tokyo is [wa]-ed but not [ni]-ed?
Is it allowed to say the same idea in English manner like this?

東京に今雨は降っている。
東京では今雨は降っている。

literally: In Tokyo now rain is falling.

Or maybe rain always must be with [ga] and never with [wa]?

To my mind I thought that what is marked as [wa] is the acting object of the verb at the end of sentence. If here verb is [to fall], then to my mind [wa]-ed Tokyo must be falling.

Or maybe the key to this situation is that the real verb here is not 降る, but maybe the real verb in this sentence is 雨が降る? And therefore 雨が never should be separated from 降る and they should stand always together like "husband and wife"?

Answer 1

While I'm not able to pin down your focus in the question, I guess you're having trouble understanding the function of は, in grammar and in mental model. You may have already heard about that は marks topic, which is not on the same level with subject, verb (predicate), or object. What does it actually mean?

Think of a theater, where actors play as they like on the stage. Interactions between actors can be described as sentences using grammatical relations like subject and object. When you say English sentences require a subject, it means that you can't make a sentence until you mention at least one actor. By the way, actors can't perform in the middle of nothing. They can't fight in the void like gods before the world creation. There have to be an implicit scene, a backdrop, a stage. A Japanese sentence requires a topic, that means, you must set up a scenery before you talk about who did what. What becomes topic is marked by は.

Of course, は is not always detached from actors. You can take out one of them and put the one as a background, like a king on the throne in the middle of the stage but does nothing, just signifying they're in the palace now. That makes an element is a topic and a participant at the same time. Such things are marked by appropriate particle + は: には, では, とは, からは etc. Note that が (≈ subject) and を (≈ object) are eaten by は, so when you see a bare は, you must suspect three possibilities: が + は, を + は, and the isolate は.

So,

literally: Tokyo now rain is falling.

No, it's like "Tokyo​: now rain is falling."

東京に今雨は降っている。

This is a grammatical sentence but not the most natural one. に used in this manner is archaic or poetic, not in the regular language. は put in the middle of sentence could have various special connotations, such as saying "not snowing but raining" or "raining it is".

東京では今雨は降っている。

The default interpretation of this sentence is, I guess, "It is indeed raining in Tokyo".

If I'd like to tell the meaning in the English-compatible way, I'd say:

東京では今雨が降っている。

Answer 2

Or maybe rain always must be with [ga] and never with [wa]?

If you want to simply say "it's raining", you have to use が. But you can use 雨は in the following situations:

• You really want a contrastive meaning, e.g. 雨は降っているが雪は降っていない.
• You refer to a certain rainfall episode that has been already brought into the universe of discourse, e.g., 雨は止んでいます ("The rain has stopped").

From here, let's just use が and compare these sentences:

1. 東京は雨が降っている。
2. 東京に雨が降っている。
3. 東京には雨が降っている。
4. 東京で雨が降っている。
5. 東京では雨が降っている。

There sentences are all correct. The subject is 雨, not 東京. 東京 is a topic when there is は (1, 3, 5). Here, I think に is a direction/destination marker rather than a location marker which can be used with いる/ある/etc (i.e., rain is falling towards the land of Tokyo). で is a location marker that marks the place where an action takes place. は is a topic marker, which can directly follow に, で, から, etc.

には can often be "contracted" to は. For example you can say both 誕生日はパーティをします, 誕生日にはパーティーをします and 誕生日にパーティーをします, but when there is は, 誕生日 is topicalized.

Related:

• What is the difference between "に" and "には"?
• What's the difference between には and では