Learn Hardcore Russian: Черепаха медленно плавает в аквариуме и иногда выглядывает из‑под камня. - The turtle swims slowly in the aquarium and sometimes peeks out from under a rock.

Черепаха is in the nominative case because it is the subject of the sentence – the thing that is doing the actions (swimming and peeking out).
Russian changes the noun ending depending on its function:

черепаха – nominative (subject: the turtle does something)
черепаху – accusative (direct object: you see / buy / feed the turtle)
черепахой – instrumental (with/by the turtle)

Here we are saying what the turtle does, so nominative черепаха is required.

Why is плавает used and not плывёт?

Russian has pairs of motion verbs, like плавать / плыть.

плавать (multi‑directional, imperfective) is used for:
- habitual/repeated actions: “The turtle swims (in general).”
- movement in different directions, back and forth, around a space.
плыть (uni‑directional, imperfective) is used for:
- one specific trip in one direction: “The turtle is swimming (from A to B).”

In an aquarium, the turtle is generally moving around in different directions, and the sentence describes a typical behavior, so медленно плавает is natural.
Медленно плывёт would sound more like “is slowly proceeding/swimming (somewhere in one direction).”

Why is it в аквариуме and not в аквариум?

After в, Russian uses:

the accusative case for direction / into: в аквариум – “into the aquarium”
the prepositional case for location / in: в аквариуме – “in the aquarium”

The sentence describes where the turtle swims (inside, as a static location), not the movement into that place. So the prepositional в аквариуме is correct: “swims in the aquarium.”

Why does аквариум change to аквариуме?

Аквариум is a masculine noun; in the prepositional case singular, the usual ending is ‑е:

nominative: аквариум
prepositional: в аквариуме

The prepositional case is triggered here by the preposition в with a location meaning (“in”). So you get в аквариуме.
This is the same pattern as дом → в доме, город → в городе.

Why is медленно before плавает? Can you say плавает медленно?

Both черепаха медленно плавает and черепаха плавает медленно are grammatically correct.

Медленно плавает (adverb before the verb) sounds very neutral and common.
Плавает медленно (adverb after the verb) slightly emphasizes how she swims, the slowness.

So word order is flexible; it’s more about nuance and emphasis than right vs. wrong.

What is the tense and aspect of плавает and выглядывает?

Both плавает and выглядывает are:

present tense
imperfective aspect
3rd person singular

Imperfective aspect here shows:

ongoing or general processes (she slowly swims)
repeated/typical actions (she sometimes peeks out)

So the sentence describes the turtle’s usual behavior, not a single one‑time event.

What does выглядывает literally mean, and why not use something like смотрит?

The verb выглядывать comes from глядеть (“to look”) with the prefix вы‑ (“out”).
Literally, it is “to look out,” but in practice it often means:

“to stick/peek out (from somewhere)” – usually with a head, face, or part of something showing.

So выглядывать из‑под камня paints a picture: the turtle’s head or body is partially hidden and then appears from under the stone.
Смотрит из‑под камня would focus more on looking with her eyes, not on the physical movement of peeking out.

Why is it из‑под камня and not just из камня or под камнем?

Russian distinguishes these:

под камнем – “under the stone” (location)
из‑под камня – “from under the stone” (movement out from that location)
из камня – “made of stone” or “out of the stone (material/source)”

The turtle is sticking/peeking out from under the stone, so из‑под (“from under”) is the correct preposition.
Из‑под is almost always followed by the genitive case.

Why does камень change to камня after из‑под?

Камень is a masculine noun. After из‑под, you must use the genitive case:

nominative: камень
genitive: камня

The preposition из‑под (“from under”) always takes the genitive, so it forces the form камня.
This is similar to из‑под стола (from under the table), из‑под кровати (from under the bed).

Why is there a hyphen in из‑под?

Из‑под is a fixed compound preposition formed from из (“from”) + под (“under”), but in modern Russian it is treated as one unit with its own meaning “from under.”
Because of that, it is written with a hyphen: из‑под.

Writing из под камня (three separate words) is considered a spelling mistake in standard Russian.

Why is it иногда выглядывает, and could you place иногда somewhere else?

Иногда means “sometimes” and is an adverb of frequency. In this sentence:

и иногда выглядывает из‑под камня is neutral and natural: “and sometimes peeks out from under the stone.”

You can move иногда a bit without changing the meaning much:

и она иногда выглядывает из‑под камня (if you include она)
и выглядывает иногда из‑под камня

The version in the sentence is a very typical placement: the adverb comes right before the verb it modifies.

Why is there no word for “the” in черепаха медленно плавает…?

Russian does not have articles (a/an/the) at all.
Whether черепаха means “a turtle” or “the turtle” is decided by context, not by a special word.

In this sentence, if the context is “our pet turtle in the aquarium,” черепаха will naturally be understood as “the turtle.”
If it’s the first mention with no prior context, it can be understood as “a turtle.”