Тебе нужно ответить на письмо до обеда.

You need to reply to the letter before lunch.

Breakdown of Тебе нужно ответить на письмо до обеда.

• письмо: the letter
• на: to
• обед: the lunch
• ты: you
• нужно: to need
• ответить: to reply
• до: before

Questions & Answers about Тебе нужно ответить на письмо до обеда.

What is the function and case of the word Тебе in this sentence?

Тебе is the dative form of ты. In the impersonal construction with нужно, the person who needs to do something is put into the dative case. Literally, “to you it is necessary to answer the letter.”

Why is нужно used here, and how does it compare to надо, должен, or следует?

All these words express necessity or obligation, but with slight nuances:

• нужно is neutral and impersonal, feels a bit softer than надо.
• надо is more colloquial and also impersonal.
• должен agrees with the subject (“Ты должен…”) and can sound stronger or more formal.
• следует is more formal and often found in written advice.
  Here, Тебе нужно… simply means “You need to…” in a neutral, everyday tone.

Why is ответить in the infinitive, and why choose the perfective aspect here?

The infinitive (ответить) follows нужно to show the action that is necessary. It is in the perfective aspect because the speaker wants to emphasize completing the action (to give a finished reply) before lunch. If you used the imperfective отвечать, it would focus on the process of replying rather than the completion.

Why is there на before письмо, and what case does письмо take?

The verb ответить (in the sense of “to reply to something”) governs the preposition на + accusative. So письмо is in the accusative case (it looks the same as nominative for neuter nouns): you reply to the letter → ответить на письмо.

What case follows до, and what exactly does до обеда mean?

The preposition до requires the genitive case, so обеда is the genitive singular of обед. До обеда literally means “up to lunch,” i.e. “by lunchtime” or “before lunch.”

Is the word order fixed? Could I say Нужно тебе ответить на письмо до обеда or Тебе до обеда нужно ответить на письмо?

Russian word order is relatively flexible, especially in impersonal constructions.

• Нужно тебе ответить… is possible and just shifts emphasis to the necessity.
• Тебе до обеда нужно ответить… works too; it stresses the time frame.
  However, the original Тебе нужно ответить на письмо до обеда is the most neutral, typical phrasing.

Can тебе be omitted? What happens if I just say Нужно ответить на письмо до обеда?

Yes, you can omit тебе in contexts where the listener is obvious:

• Нужно ответить на письмо до обеда.
  This still means “Someone (you) needs to reply to the letter before lunch.” It becomes more impersonal or general. Adding тебе personally addresses “you.”