Learn Hardcore Japanese: asita ha hayaku kaereru to omoimasu.

Using は with a time word like 明日 is very common when you’re contrasting it with other times (e.g., Today I can’t go home early, but as for tomorrow…), or just when you want to make tomorrow the frame of what you’re talking about.

You could also say 明日早く帰れると思います (without は). That is still understandable, but it sounds a bit more like you just listed 明日 as one of several pieces of information, without highlighting it as the main topic. With は, 明日 feels slightly more “in focus.”

Where is the word I in this sentence? Why is it not written?

Japanese often omits the subject when it’s clear from context.

In 明日は早く帰れると思います, the default interpretation is:

(私は) 明日は早く帰れると思います。
= (I) think I can go home early tomorrow.

私は (I + topic marker) is not written because:

The verb 思います (think) naturally suggests it’s the speaker’s thought.
The action 帰れる (can go home) also naturally fits the speaker unless context clearly points to someone else.

So you don’t need to say 私 unless you want to emphasize it or avoid ambiguity. This kind of omission is completely normal in Japanese.