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The sentence

> Ashita **wa** ame ga furu deshou

uses a "wa" after _ashita_. But the sentence

> Ashita kaerimasu

doesn't. Why is that? Is it being omitted or would it be wrong to say

> ashita **wa** kaerimasu

?
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question
particle-は
Why is "ashita kaerimasu" not "ashita wa kaerimasu"? is the particle wa being omitted or is it not necessary?
> **あした{ashita}は{wa}** あめ{ame}が{ga} ふ{fu}る{ru}で{de}しょう{syou}。

It has は because あした is the theme/subject of the predicate/sentence.

> **あした** 、かえり{kaeri}ます{masu}。

Here, あした is an adverb. It's just saying 'when.'

If you say

> あした **は** かえります

あした becomes the theme/subject of the sentence. You can say this, too. It's no
problem. Just it sounds like comparing with something, maybe comparing with
きょう (today), or きのう (yesterday).

* * *

[Reply to additional request]

あした、かえります sounds simple; ex. "I'll be/go/come back/home tomorrow."

あしたはかえります most likely does not say someone or something named あした goes back.
The action maker of かえります is someone you can tell from the context. It could
be "(Sorry that I couldn't leave here by now, but) I'll come back tomorrow" or
"(I enjoyed the sightseeing today.) Tomorrow, I'm going home."
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answer
First thing you should know: "は" particle is a topic-marker particle used to
form a statement. The comprehensive usage already explained
[here.](https://japanese.stackexchange.com/questions/22/whats-the-difference-
between-wa-%e3%81%af-and-ga-%e3%81%8c)

Given first example you have:

> 明日は雨が降るでしょう (It will rain tomorrow)

Here a noun (雨) and particle-が is used, then 明日 is a subject of the sentence
which has particle-は before the noun as topic marker.

And the second example:

> 明日帰ります

Here "明日" used as adverb, there is unnecessary to use another "は". But if you
want to place "明日" as a subject (i.e. for emphasis), you can add particle-は
between the subject & verb:

> 明日は帰ります

Both are having same " _I will return tomorrow_ " meaning. However the former
structure is more often used, especially when combined with other subject:

> パブロは明日帰ります。(Pablo will return tomorrow)

Here are some adverbs which have similar traits like 明日 or 昨日 (kinō,
yesterday):

>   * 去年 (kyonen)
>   * 去月 (kyogetsu)
>   * 一昨日 (ototoi)
>   * 来週 (raishū)
>   * 来月 (raigetsu)
>   * 来年 (rainen)
>
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answer