osoraku asita ha ame desu.

You’re right: normal Japanese writing does not use spaces between words.

So this sentence would usually be written as:

おそらく明日は雨です。

Spaces are often added in beginner materials to make the parts easier to see. They are just a learning aid, not standard writing.

おそらく is an adverb meaning something like probably, most likely, or presumably.

In this sentence, it modifies the whole statement:

• おそらく明日は雨です。
• Tomorrow will probably be rainy / It will probably rain tomorrow.

Compared with たぶん:

• たぶん is very common in everyday speech and sounds a bit more conversational.
• おそらく can sound a little more formal, written, or deliberate.

So both can work, but:

• たぶん明日は雨です。 = very natural in casual conversation
• おそらく明日は雨です。 = understandable, slightly more formal/stiff

は marks the topic of the sentence.

So 明日は means:

• as for tomorrow
• speaking of tomorrow

The sentence is setting tomorrow as the topic, and then saying something about it:

• 明日は雨です。
• As for tomorrow, it’s rain / it will be rainy.

In natural English, we usually translate that simply as:

• It will rain tomorrow
• Tomorrow will be rainy

But grammatically, Japanese is not using tomorrow exactly as the subject in the same way English does. It is the topic.

Because おそらく is an adverb.

Adverbs usually modify a sentence or verb directly and do not need particles like は, が, or を.

So:

• おそらく = probably
• 明日 = tomorrow
• は = topic marker
• 雨 = rain
• です = polite copula

Here, おそらく simply adds the meaning probably to the whole statement.

Yes, 雨 is a noun meaning rain. But in Japanese, weather expressions often use a noun + です.

So:

• 雨です literally looks like it is rain
• but naturally means it is rainy or it will rain

This is very common with weather:

• 今日は雨です。 = It’s rainy today / It’s raining today
• 明日は雪です。 = It will snow tomorrow / Tomorrow will be snowy

Japanese often expresses weather with a noun where English prefers an adjective or verb.

There could be. Japanese has other natural ways to express the same idea.

For example:

• 明日は雨です。 = Tomorrow will be rainy / It will rain tomorrow.
• 明日は雨が降ります。 = Rain will fall tomorrow / It will rain tomorrow.

The version with 雨です is shorter and very common in weather forecasts or general statements about weather.

The version with 雨が降ります focuses more on the action of rain falling.

Both are correct; they just phrase it differently.

This is a very common beginner question.

Japanese non-past forms often cover both:

• present
• future

So です here is not specifically “present tense only.” Since the sentence includes 明日 (tomorrow), the time reference is clearly future.

So:

• 明日は雨です。 means
• Tomorrow will be rainy / It will rain tomorrow

Japanese usually relies on context or time words like 明日, 来週, あとで, etc. to show future time.

Japanese often omits subjects when they are obvious or unnecessary.

English needs a subject in a sentence, even a dummy subject like it in weather expressions:

• It is raining
• It will rain

Japanese does not need that kind of placeholder subject. So the sentence can simply be:

• おそらく明日は雨です。

There is no separate word for it here, and that is completely normal.

Normally, 明日は雨です is the natural choice.

Why?

Because 明日 is being presented as the topic: as for tomorrow.

Using が after 明日 would usually sound unnatural in this sentence. が tends to mark a subject in a more specific or contrastive way, and 明日が雨です is not the standard phrasing for a plain weather statement.

So for a basic sentence like this, use:

• 明日は雨です。

not:

• 明日が雨です。

Yes, it is grammatical and understandable. But depending on context, it may sound a little stiff or formal.

Some very natural alternatives are:

• 明日はたぶん雨です。
• 明日は雨でしょう。
• 明日はたぶん雨でしょう。

Nuance:

• たぶん = common, conversational probably
• でしょう = probably / I expect / it seems likely

So while おそらく明日は雨です。 is fine, many speakers would more naturally say something like:

• 明日はたぶん雨です。 or
• 明日は雨でしょう。

Yes. Japanese word order is more flexible than English, especially with adverbs like おそらく.

These are both possible:

• おそらく明日は雨です。
• 明日はおそらく雨です。

The meaning is basically the same: Tomorrow will probably be rainy.

That said, different placement can slightly change what feels emphasized:

• おそらく明日は雨です。
  starts with probably, so the uncertainty comes first
• 明日はおそらく雨です。
  starts with tomorrow, so the time is introduced first

Both are understandable and natural enough, depending on style and context.