Learn Hardcore Russian: У клетки с тигром мы читаем, чем это животное питается и сколько часов спит. - At the cage with the tiger we read what this animal eats and how many hours it sleeps.

у always requires the genitive case, so клетка → клетки.
Without у, клетка is just “a cage” (nominative, subject), and клетке (dative / prepositional) would need a different preposition or verb.

So у клетки is a set expression: у + genitive = “by / at / near something.”

What exactly does у клетки с тигром mean as a whole?

У клетки с тигром = “by the cage with the tiger” / “at the tiger’s cage.”

у клетки – “by the cage”
с тигром – “with a tiger (in it)”

Put together, it describes a location: you are standing next to the cage that has a tiger inside.

Why is it с тигром and not just тигр or тигра?

С тигром uses the instrumental case after с, which here means “with, containing.”

клетка с тигром = “a cage with a tiger in it.”
клетка тигра (genitive) would be “the tiger’s cage” (ownership), not “a cage that currently contains a tiger.”

So с тигром describes what is inside the cage, not who owns it.

Could the sentence start with Мы instead of У клетки с тигром? Does the word order matter?

You could say: Мы читаем у клетки с тигром, чем это животное питается… and it’s grammatically fine.

Russian often puts the place at the beginning to set the scene:

У клетки с тигром мы читаем… = “At the tiger’s cage, we read…”

The meaning is the same; the given word order just emphasizes where this happens.

Why is there a comma before чем: мы читаем, чем это животное питается…?

The comma separates the main clause from a subordinate clause:

Main clause: мы читаем – “we read”
Subordinate clause: чем это животное питается… – “what this animal eats…”

In Russian, a clause introduced by words like что, как, когда, чем, сколько after a verb like читать, знать, видеть is normally set off with a comma.

Why is it чем это животное питается and not что это животное ест?

Both are possible, but they’re not identical grammatically:

есть что – “to eat something” (direct object, accusative)
питаться чем – “to feed on something / to live on something” (instrumental)

Here the sentence uses the verb питаться (“to feed on, be nourished by”), which requires the instrumental case, so the interrogative word must also be instrumental:

что? (accusative) → with есть
чем? (instrumental) → with питаться

So чем это животное питается = “what this animal feeds on.”

What does the verb питается mean exactly, and why does it have -ся?

Питается is the 3rd person singular present of питаться.

Base verb: питать – “to nourish, to feed (someone/something)”
питаться (with -ся) – “to feed (oneself), to live on, to eat (as a diet)”

The -ся here makes the verb reflexive / middle voice: the animal is feeding itself with something.
It always takes the instrumental: питаться мясом, травой, рыбой.

Who is the subject of спит in и сколько часов спит? There is no noun there.

The subject is still это животное from the previous clause. Russian often omits a repeated subject if it’s clear from context.

You can imagine the full version as:

…чем это животное питается и сколько часов (это животное) спит.

In English we must repeat “it”, but in Russian it’s normal to drop it.

What tense/aspect are питается and спит? Do they mean “is eating / is sleeping now”?

Both are present tense, imperfective aspect. In this context, they express general, habitual facts, not actions happening right now:

питается – “feeds on / eats (as a rule)”
спит – “sleeps (for X hours, typically)”

So the idea is “what this animal typically eats and how many hours it typically sleeps.”

How does сколько часов спит work grammatically? Why isn’t there a preposition like “for”?

Сколько часов спит literally is “how many hours (it) sleeps.”

Structure:

сколько – “how many / how much”
часов – genitive plural of час (“hour”) after сколько
спит – “sleeps”

Russian doesn’t need a preposition “for” the way English does. Duration with verbs like спать, ждать, работать is often just [сколько] + [time in genitive] + [verb]:

Он спит три часа. – “He sleeps for three hours.”

Why do we say это животное instead of repeating тигр, and why is it neuter?

Животное is a neuter noun meaning “animal”, and это agrees with it in gender and number: это животное = “this animal.”

Possible reasons to use it instead of тигр:

stylistic variety (avoid repeating тигр immediately)
slightly more neutral / encyclopedic tone, like on an information board: “this animal feeds on…”

Even though тигр is masculine, животное is neuter by its own grammatical gender, so you get это животное, оно питается, оно спит.

What is мы читаем reading exactly? There is no direct object like “text” or “sign.”

The object of читаем is the whole set of subordinate clauses:

мы читаем, чем это животное питается и сколько часов спит
= “we read what it eats and how many hours it sleeps.”

In Russian, verbs like знать, понимать, помнить, читать (in this sense) can take a clause introduced by что, как, когда, чем, сколько as their object, without any extra word like “the fact that” or “information about.”