Learn Hardcore Portuguese (Brazil): Por favor, devolva o guarda-chuva amanhã cedo. - Please return the umbrella early tomorrow.

Devolva is the affirmative imperative form of the verb devolver used with você (third-person singular). Technically, it comes from the present-subjunctive stem (devolva-) and functions as a polite command: “(You) return.”

Why is devolver used instead of voltar?

Because devolver means “to give back” or “to return an object,” while voltar means “to return/go back” in the sense of going back to a place. To ask someone to hand an item back, you need devolver.

Why is there a hyphen in guarda-chuva and why do we say o guarda-chuva?

Guarda-chuva is a compound noun (“guard of rain”), so Portuguese orthography keeps it hyphenated.
It’s grammatically masculine, so the definite article is o: o guarda-chuva = “the umbrella.”

What does amanhã cedo mean and how is it different from amanhã de manhã?

Amanhã cedo literally means “early tomorrow,” focusing on the early part of the next day (usually early morning).
Amanhã de manhã means “tomorrow morning,” without emphasizing how early.

Use amanhã cedo when you want someone to do something as soon as the next day begins.

Can I say cedo amanhã or amanhã de manhã cedo instead of amanhã cedo?

Cedo amanhã is not idiomatic in Portuguese.
Amanhã de manhã cedo is grammatically correct but wordy.
The most natural expressions are amanhã cedo or, if you want to specify the morning more broadly, amanhã de manhã.

How would I explicitly say “return it to me” in Portuguese?

You introduce the indirect-object pronoun me. In an affirmative imperative you can attach it or place it before, for example:

Formal/enclisis: Por favor, devolva-me o guarda-chuva amanhã cedo.
Colloquial/proclisis (common in Brazil): Por favor, me devolva o guarda-chuva amanhã cedo.

Is the comma after Por favor necessary? Can I move por favor elsewhere?

The comma after Por favor simply marks a pause and is optional. You can also say:

Por favor devolva o guarda-chuva... (no comma)
Devolva o guarda-chuva amanhã cedo, por favor. (politeness at the end)

Can I start the sentence with Amanhã cedo?

Yes. Portuguese word order is flexible with time expressions and polite particles. For instance:
Amanhã cedo, por favor devolva o guarda-chuva.

How do you pronounce amanhã and guarda-chuva, and what does the tilde do?

amanhã is pronounced approximately ah-ma-NYAH̃, with the tilde on ã indicating nasalization.
guarda-chuva is GWAR-da SHOO-va, where ch makes a “sh” sound. In Brazilian Portuguese, the r in guarda is a guttural /ʁ/.