La mesa está contra la pared, y la silla queda junto a ella.

Because Spanish normally uses estar for location.

• La mesa está... = the table is located...
• Es would be used for identity or essential description, such as La mesa es redonda.

So for where something is, estar is the normal choice.

Here contra la pared means against the wall.

It usually suggests that the table is placed so that it is touching the wall, or at least pressed right up to it. In this sentence, contra is physical, not figurative.

Also, unlike a + el or de + el, contra does not form a contraction with the article:

• contra la pared
• contra el muro

In this sentence, queda means something like is positioned, ends up being, or sits.

In Spain, quedar is often used to describe where something is in relation to something else, especially when talking about arrangement or placement.

So:

• La silla queda junto a ella = the chair is positioned / sits next to it
• La silla está junto a ella would also be correct, but it sounds more neutral and straightforward

So queda adds a slight nuance of placement rather than just basic location.

Not exactly.

The basic verb quedar can mean to remain, to be left, or to end up. But in sentences like this one, it often has a more natural English sense like:

• is
• is located
• sits
• lies

So although queda can sometimes mean stays, here it is better understood as is positioned or is located.

Junto a means next to or right beside.

It is very close in meaning to al lado de. In many everyday contexts, they are interchangeable:

• La silla queda junto a la mesa
• La silla queda al lado de la mesa

Both mean the chair is next to the table.

A small difference:

• junto a can sound a little more formal or descriptive
• al lado de is often the most direct everyday way to say next to

In Spain, junto a is completely natural.

Grammatically, ella is a feminine singular pronoun, so it could refer to either:

• la mesa
• la pared

Both nouns are feminine singular, so the sentence is technically ambiguous if you look only at grammar.

Usually, context tells you which one is meant. If the speaker wants to be completely clear, they might repeat the noun instead:

• La silla queda junto a la mesa
• La silla queda junto a la pared

Because Spanish only has two standard contractions:

• a + el = al
• de + el = del

There is no contraction with pronouns such as ella, él, ellos, etc.

So:

• junto a ella = correct
• junto al would only be possible with el as an article, as in junto al sofá

Spanish uses the definite article more often than English does.

In a sentence like this, the speaker is referring to specific, identifiable objects in the scene, so la is natural:

• la mesa = the table
• la pared = the wall
• la silla = the chair

Also, every noun in Spanish has grammatical gender, so the article shows that too:

• mesa = feminine
• pared = feminine
• silla = feminine

Usually, no.

In standard Spanish, you normally would not put a comma before y when simply joining two related clauses like these. A more typical version would be:

La mesa está contra la pared y la silla queda junto a ella.

The comma is not impossible, but it is less neutral here. It may be used to create a pause or a certain rhythm, but most learners should avoid adding it in simple cases like this.

Yes, absolutely.

That version is:

• grammatically correct
• more direct
• less ambiguous than junto a ella

Compare:

• La silla queda junto a ella = more descriptive, but ella could be ambiguous
• La silla está junto a la mesa = simpler and clearer

So if you want to avoid any confusion, repeating the noun is often the best choice.