Learn Hardcore French: Rends-moi le livre tout à l'heure, d'accord?

It’s the imperative (a direct request/command). For rendre, the imperative forms are: rends (tu), rendons (nous), rendez (vous).
In affirmative imperatives, object pronouns go after the verb with a hyphen, and me becomes moi: Rends-moi …
Me rends … would be indicative (not an order) and would need a subject: Tu me rends …

Why is it moi instead of me?

In the affirmative imperative, me/te change to moi/toi and follow the verb with a hyphen: Rends-moi.
In the negative imperative, they go back to me/te in front of the verb (no hyphen): Ne me rends pas …

If I replace le livre with “it,” how does the sentence change?

Use pronouns and keep their order: Rends-le-moi tout à l’heure, d’accord?
Order in affirmative imperatives: verb – le/la/les – moi/toi/lui/nous/vous/leur – y – en (hyphens between all parts).

Where do the hyphens go? Any changes with y or en?

Affirmative imperative: hyphens between the verb and each pronoun: Rends-le-moi, Donne-m’en.
With y or en, moi/toi become m’/t’: Donne-m’en, Vas-y.
For -ER verbs, the tu form normally drops the -s (e.g., Parle !), but it returns before y/en: Parles-en !, Vas-y !
Negative imperative: no hyphens, pronouns before the verb: Ne me le rends pas.

What exactly does tout à l’heure mean here?

It means later today / in a little while (soonish, usually the same day). It’s vaguer than a precise time and often softer than plus tard.
For precision, say dans une heure, cet après-midi, ce soir, etc.

Can tout à l’heure also mean “earlier”?

Yes. In a past context it means a little earlier. Example: Je l’ai vu tout à l’heure = I saw him a short while ago.
Context or verb tense tells you whether it’s future or past.

Can I put tout à l’heure somewhere else in the sentence?

Yes. Tout à l’heure, rends-moi le livre, d’accord? is fine. End position is the most common.

Is Rends le livre à moi correct?

It’s grammatical but unnatural in most contexts. Say Rends-moi le livre or (with pronouns only) Rends-le-moi.
À moi is used for emphasis/contrast: Rends-le à moi, pas à Paul.

What’s the difference between rendre, donner, redonner, retourner, ramener/rapporter?

rendre: return something to its owner (the best verb here).
redonner: give again; often used to mean “give back,” but it emphasizes the repetition.
retourner: return/send back to a place (e.g., a store), not to a person.
ramener (bring back a person) / rapporter (bring back a thing) to a place: Rapporte le livre à la bibliothèque. You can also say Rends le livre à la bibliothèque.

How can I make the request softer/more polite?

Add a softener: …, s’il te plaît / …, s’il vous plaît.
Use a question instead of an imperative:
- Tu peux me rendre le livre tout à l’heure, s’il te plaît ?
- Vous pouvez me rendre le livre tout à l’heure, s’il vous plaît ?
Or a statement with a tag: Tu me rends le livre tout à l’heure, d’accord ?

What does d’accord? add? Are there alternatives?

d’accord? checks agreement/consent: like OK? / right?
Alternatives (register varies): OK ?, ça te va ? / ça vous va ?, c’est bon ?, hein ? (very informal), n’est-ce pas ? (after a statement, not usually after an imperative).

Can I use n’est-ce pas? here?

After an imperative, n’est-ce pas ? sounds odd. Prefer d’accord ? or rephrase as a statement: Tu me rends le livre tout à l’heure, n’est-ce pas ?

Why le livre and not la livre?

Any pronunciation tips?

Rends-moi: the -ds in rends is silent; nasal vowel in rends: roughly [ʁɑ̃ mwa].
le livre: [lə livʁ].
tout à l’heure: liaison on t of tout before the vowel of à: [tu ta lœʁ].
d’accord ?: [dakɔʁ] with rising intonation.

Any punctuation/spelling gotchas?

Keep the hyphen in Rends-moi.
Don’t forget accents/apostrophes: tout à l’heure, d’accord, à l’heure.
Traditional French typography uses a thin space before ?, but many people omit it informally.

How does the sentence look in the negative?

Ne me rends pas le livre tout à l’heure. (Pronoun before the verb; no hyphens.)
With two pronouns: Ne me le rends pas tout à l’heure.

Is there a softer non-imperative version with the same meaning?

Yes, very common in speech: Tu me rends le livre tout à l’heure, d’accord ? It states the plan and asks for confirmation, sounding less brusque than the imperative.