мне нужно / надо

The construction follows the formula: [Dative Experiencer] + [нужно / надо] + [Infinitive].

1. Dative Experiencer:
The person who needs something must be in the Dative case, not Nominative (never say 'Я надо' to mean 'I need'). The Dative pronouns are: мне, тебе, ему, ей, нам, вам, им. Note that 'Я' can be the Nominative subject when using short adjectives (e.g., 'Я тебе нужен?' - 'Do you need me?').

2. Нужно vs. Надо:
While often interchangeable, надо is more common in casual speech and can feel more urgent or subjective. Нужно is slightly more formal and neutral, often implying an objective necessity.

3. Tense Markers:
Because this is an impersonal construction, the verb 'to be' (быть) is omitted in the present tense (zero copula), and appears in the fixed neuter singular form in the past and future:

• Present: Мне надо идти. (I need to go.)
• Past: Мне надо было идти. (I needed to go.)
• Future: Мне надо будет идти. (I will need to go.)

4. Negation and Aspect:
Adding не before the modal word changes the meaning. Не надо often acts as a prohibition ('don't') or strong advice against an action, while не нужно usually means 'it is not necessary.' Crucially, negated necessity (не надо / не нужно) strictly requires an imperfective infinitive, even for one-time actions. In the past and future tenses, the tense marker comes after the negated modal word (e.g., Мне не надо было идти - 'I didn't need to go' / Мне не нужно будет ждать - 'I won't need to wait').

5. General Necessity:
To express a general requirement without specifying who needs to do it, simply omit the dative experiencer (e.g., Нужно подождать - 'One needs to wait' or 'It is necessary to wait').

6. With Nouns vs. Verbs:
In standard, formal Russian, use the invariable modal words надо/нужно only with verbs (infinitives). If you need a noun (e.g., 'I need a book'), the strict rule is to use the short adjective forms (нужен, нужна, нужно, нужны) which agree with the thing needed, as it becomes the grammatical subject in the Nominative case. Note that the invariable word нужно used with verbs and the neuter short adjective нужно used with neuter nouns (like Мне нужно время) look and sound exactly the same. When using these short adjectives in the past or future tense, the tense marker (быть) must also agree with the noun's gender and number (e.g., Мне нужна была книга, Мне нужны будут деньги). This contrasts with verbs, where было/будет is strictly fixed in the neuter singular regardless of the speaker. However, in everyday colloquial speech, you will frequently hear надо + noun (e.g., Мне надо новую работу), where the noun takes the Accusative case (or Genitive). Additionally, when negating a needed noun, Russian frequently uses the Genitive case with the neuter нужно or надо (e.g., Мне не нужно проблем).