Si resistes la tentación hoy, te sentirás mejor mañana.

Spanish uses different “if”-clauses depending on how real or hypothetical the situation is.

• For a real or likely condition, you use si + present indicative in the first clause and future indicative in the second:
  • Si resistes la tentación…, te sentirás mejor.
• You never put the future in the si clause (e.g. Si resistirás … is incorrect).
• For a more hypothetical idea, you’d say si + past subjunctive and conditional:
  • Si resistieras la tentación, te sentirías mejor.

The verb sentirse is pronominal, which means “to feel (a certain way).”

• Te is required because you’re describing how you feel.
• Without te, sentirás would be transitive (“you will feel something”), not reflexive (“you will feel yourself in a better state”).

Bien is an adverb meaning “well,” but mejor is a comparative adjective/adverb meaning “better.”

• Te sentirás bien = “you will feel well/good.”
• Te sentirás mejor = “you will feel better (than you feel now).”
  Since the sentence compares how you’ll feel tomorrow vs. today, mejor is the natural choice.

It’s all about Spanish stress rules:

• Words ending in a vowel, “n” or “s” are normally stressed on the second-to-last syllable.
• Sentirás ends in “s” but is stressed on the last syllable (sen-ti-RÁS), so it needs an accent to override the default rule.
• Resistes ends in “s” and is naturally stressed on the penultimate syllable (re-SIS-tes), so no written accent is needed.

When a subordinate clause starts the sentence, you must separate it from the main clause with a comma:

• Correct: Si resistes la tentación hoy, te sentirás mejor mañana.
• If you reverse the order (main clause first), the comma before the si clause becomes optional:
  • Te sentirás mejor mañana si resistes la tentación hoy.

Yes—they function as adverbs of time, not as nouns.

• As adverbs, hoy (“today”) and mañana (“tomorrow”) modify the verbs directly and never need an article.
• If you treat mañana as a noun (“the morning” or “the future”), you would add an article: el mañana, but that changes the meaning.

Yes, you have choices:

• Futuro simple (te sentirás) is direct and a bit more formal: “you will feel.”
• Futuro perifrástico (vas a sentir) is colloquial and emphasizes immediacy: “you’re going to feel.”
• Conditional (te sentirías) would imply a more hypothetical or polite tone: “you would feel,” which doesn’t match the real-future sense of this sentence.