Под скамейкой сидел паук, а в углу уже висела старая паутина.

Because под can take different cases depending on meaning:

• под + instrumental = location, under something
• под + accusative = movement to a position under something

Here the spider is already located there, not moving there, so Russian uses the instrumental:

• под скамейкой = under the bench

Compare:

• Паук сидел под скамейкой. = The spider was sitting under the bench.
• Паук залез под скамейку. = The spider crawled under the bench.

The dictionary form is скамейка. But after под in the meaning of location, the noun must be in the instrumental singular.

For a feminine noun ending in -а, the instrumental singular is often -ой / -ей:

• скамейка → скамейкой
• лампа → лампой
• неделя → неделей

So под скамейкой is just the correct case form.

Both are possible, but the word order changes the focus.

Russian word order is more flexible than English. In this sentence, под скамейкой sets the scene first, and сидел comes before паук to present the spider as something being noticed there:

• Под скамейкой сидел паук = Under the bench, there was a spider sitting / a spider was sitting there.

This feels natural in descriptive narration. It is a bit like introducing something into a scene.

If you said Паук сидел под скамейкой, that would also be correct, but it sounds more neutral and more directly about the spider as the topic.

Угол has a special location form often used after в or на when talking about being physically in a corner:

• в углу = in the corner

This is a special old locative-type form that some masculine nouns have.

So:

• угол = corner
• в углу = in the corner

You may also encounter в угле, but в углу is the usual form when talking about physical location in a corner of a room.

Because Russian uses в for being inside a space or area, and a corner of a room is viewed as a location in that space:

• в углу = in the corner

English also says in the corner, so this matches quite well here.

Уже means already, but in Russian it often adds a sense that something is already there by that time or has been there for some time.

So:

• а в углу уже висела старая паутина

suggests that the old cobweb was not new; it had already been hanging there. It helps create atmosphere and makes the scene feel more lived-in, old, or neglected.

Because висеть means to hang. A cobweb is something that hangs, so this verb is more vivid and natural than a more general verb like была.

• висела паутина = a cobweb was hanging
• была паутина = there was a cobweb

Both can be understandable in context, but висела is much more natural here because it describes how the cobweb was positioned.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is паутина, which is a feminine singular noun. So the verb must also be feminine singular:

• masculine: висел
• feminine: висела
• neuter: висело
• plural: висели

So:

• старая паутина висела
• not висел, because паутина is feminine

Because the subject is паук, and паук is a masculine noun.

Russian past tense agrees with the subject:

• паук сидел = the spider was sitting
• кошка сидела = the cat was sitting
• животное сидело = the animal was sitting

So сидел is masculine singular, matching паук.

Because adjectives in Russian must agree with the noun in gender, number, and case.

Паутина is feminine singular nominative here, so the adjective старый becomes старая:

• masculine: старый
• feminine: старая
• neuter: старое
• plural: старые

So:

• старая паутина = old cobweb

Both words match each other grammatically.

Russian does not have articles like English a and the.

Whether something means a spider, the spider, an old cobweb, or the old cobweb depends on context, word order, and what is already known in the conversation.

So:

• сидел паук can mean a spider was sitting
• in another context, it could also mean the spider was sitting

Here, because the spider is being introduced into the scene, English naturally translates it as a spider.

А often means and, but not exactly the same kind of and as English in every situation.

It often connects two parts while also giving a slight sense of contrast, switch of attention, or meanwhile / whereas.

Here it works like:

• Under the bench sat a spider, and in the corner an old cobweb was already hanging.

The sentence moves the reader’s attention from one part of the scene to another. А is very natural for that kind of shift.

Because the sentence is describing a scene, not focusing on completed actions.

• сидел from сидеть = was sitting / sat
• висела from висеть = was hanging / hung

These are imperfective verbs, which are commonly used for:

• background description
• ongoing states
• scene-setting

The sentence is not about the spider finishing the action of sitting or the cobweb completing the action of hanging. It is simply describing what was there.

Yes, that is a fairly literal translation, and it helps show the Russian structure.

But in natural English, you would usually say something like:

• Under the bench, a spider was sitting, and in the corner an old cobweb was already hanging.
• or more naturally:
• A spider was sitting under the bench, and an old cobweb was already hanging in the corner.

Russian often uses word order more freely for emphasis and scene-building, so the most literal English version may sound a bit literary or dramatic.

In Russian, паутина can mean spiderweb / cobweb as a material or a web. In context, it often refers to a single visible web.

So старая паутина here naturally corresponds to an old cobweb in English.

English often prefers the countable word cobweb in this kind of scene, while Russian comfortably uses паутина.

So this is more about natural translation than strict grammar.