Learn Hardcore Russian: После долгой прогулки мне нужно поесть что‑нибудь лёгкое. - After a long walk I need to eat something light.

Literally: после долгой прогулки = “after (of) a long walk.”
Any noun after после must be in the genitive: после работы, после урока, после обеда etc.

Why is there no word for “a” or “the” in после долгой прогулки?

Russian has no articles (no direct equivalents of “a/an” or “the”).
Definiteness or indefiniteness is understood from context:

после долгой прогулки can mean “after a long walk” or “after the long walk,” depending on what was mentioned before.

You simply use the correct case form (долгой прогулки – genitive) and context does the rest.

What does мне нужно literally mean, and how is it built grammatically?

Literally, мне нужно is “to me it is necessary.”

мне – dative of я (“to me”)
нужно – a predicative adverb meaning “necessary / needed”

This is a very common impersonal construction: Russian often says “To me it-is-necessary to do X” instead of “I need to do X.”
So мне нужно поесть = “It is necessary for me to eat” → “I need to eat.”

What’s the difference between мне нужно and я должен / я должна?

Both can translate as “I need to / I have to,” but with different nuances:

мне нужно поесть – neutral “I need to eat” (a practical necessity, internal need, or simple requirement)
я должен поесть (male) / я должна поесть (female) – “I must / I am obliged to eat,” sounds more like a duty or obligation

In everyday speech about your own needs, мне нужно (or мне надо) is more natural than я должен.

Why is it поесть, not есть or съесть?

All three are related to “to eat,” but with different nuances:

есть – imperfective, basic “to eat” (focus on the process, habitual: “I eat, I am eating”)
поесть – perfective with a nuance “to eat for a while / to have something to eat” (start and finish the action; often “have a bite”)
съесть – perfective, “to eat up / to finish eating (something specific)”

In мне нужно поесть, the idea is “I need to have something to eat (eat a bit),” not to describe the process (есть) or emphasize finishing a particular portion (съесть).

What nuance does поесть add here compared to мне нужно есть что‑нибудь лёгкое?

Мне нужно поесть что‑нибудь лёгкое suggests:

one action of eating
probably soon, as a single event
with a slight “have a bite / get some food” nuance

Мне нужно есть что‑нибудь лёгкое would sound like a regular rule or diet: “I need to eat something light (in general / regularly).”
So поесть fits better for a one‑time need after a long walk.

Why is it что‑нибудь лёгкое and not что‑нибудь лёгкий / лёгкая?

Что‑нибудь is neuter, so any adjective describing it must also be in neuter: лёгкое.

что‑нибудь – “something / anything” (indefinite pronoun, neuter)
лёгкое – neuter singular of лёгкий (light)

So что‑нибудь лёгкое literally is “something light.”
The form лёгкий (masc.) or лёгкая (fem.) would not agree with что‑нибудь.

What noun is implied in что‑нибудь лёгкое? Why can Russian omit it?

In context, it clearly means “something light (to eat)” – usually лёгкое блюдо (a light dish) or лёгкая еда (light food).
Russian often omits a predictable noun when it’s obvious:

хочу чего‑нибудь сладкого – “I want something sweet (to eat/drink).”
буду что‑нибудь холодное – “I’ll have something cold (to drink).”

So что‑нибудь лёгкое is enough; the listener infers that it’s food, because of поесть.

Could I say что‑то лёгкое instead of что‑нибудь лёгкое? What’s the difference?

Yes, you can say что‑то лёгкое; both are grammatically correct.

Nuance:

что‑нибудь лёгкое – more open‑ended, often “anything light,” not very specific; slightly more colloquial and common in this kind of sentence.
что‑то лёгкое – “some light thing,” often implies you have some idea or there is some specific but unnamed option.

In this context, both will be understood the same; что‑нибудь лёгкое is very natural here.

What is the grammatical form of поесть in this sentence?

Поесть is the infinitive form (perfective aspect).
In the structure мне нужно + infinitive, the verb is always infinitive:

мне нужно поесть – I need to eat
мне нужно отдохнуть – I need to rest
мне нужно уйти – I need to leave

The subject “I” is understood from мне; there is no separate я in the sentence.

Can the word order be changed, for example: Мне после долгой прогулки нужно поесть что‑нибудь лёгкое?

Yes. Russian word order is relatively flexible.

All of these are grammatical, with only slight changes in emphasis:

После долгой прогулки мне нужно поесть что‑нибудь лёгкое. (neutral; sets the time first)
Мне после долгой прогулки нужно поесть что‑нибудь лёгкое. (slight focus on “for me, after a long walk…”)
Мне нужно после долгой прогулки поесть что‑нибудь лёгкое. (focus on “I need to eat something light after the walk.”)

The original version is the most typical and neutral.

Why is лёгкое written with ё? Is легкое also possible?

The correct spelling in standard Russian here is лёгкое (from лёгкий).
The sound is [о] under stress, written as ё.

In many printed texts, ё is replaced with е (легкое) for typographical reasons, but:

Pronunciation remains [лёгкое]
In learning materials and dictionaries, ё is usually written to show stress and sound clearly.

So you should pronounce it лёгкое, even if you sometimes see легкое in print.

What’s the difference between нужно and надо in this sentence? Could I say мне надо поесть что‑нибудь лёгкое?

Yes, мне надо поесть что‑нибудь лёгкое is also correct and very natural.

Nuance:

нужно – slightly more neutral or “bookish”
надо – more colloquial, very common in everyday speech

In conversation, мне надо поесть что‑нибудь лёгкое might actually be more frequent, but мне нужно… is equally acceptable.