Learn Hardcore Japanese: kabegiwa ni tiisai sofa wo okimasu.

If you said 壁ぎわでソファを置きます, it would sound like “I (somehow) do the action of placing in the area by the wall,” which is unnatural for this meaning.
For verbs of putting, existing, coming/going (置く, ある, いる, 行く, 来る, 住む, etc.), に is typically used to mark the location.

What does the particle を do after ソファ?

を marks the direct object of the verb — the thing that is being acted on.

ソファを置きます。
- ソファ = sofa
- を = object marker
- 置きます = place / will place

So it literally means: (I) will place the sofa.

If you changed を to が, it would mean “the sofa places (something)” (i.e., the sofa becomes the subject) which doesn’t make sense here.
So:

X を置きます = I place X.
場所に X を置きます = I place X at 場所.

Why is there no word for I or we in the sentence?

Japanese often omits the subject when it is clear from context.

In English, we must usually say I will place, we will place, etc. In Japanese, if it’s obvious who is acting (from the situation, previous sentences, or social roles), the subject is just dropped.

So:

(私は) 壁ぎわに小さいソファを置きます。
= I will place a small sofa by the wall.

The 私は part is understood but not spoken. You would only explicitly add 私, 私たち, 彼, etc., if you need to clarify or contrast the subject.

Why is 小さい used before ソファ? Can I also say 小さなソファ? Is there a difference?

小さい is an i-adjective, and i-adjectives can directly modify nouns:

小さいソファ = a small sofa

You can also say:

小さなソファ

Here, 小さな acts like a na-adjective form of the word 小さい. Both are correct and natural.

Nuance:

小さいソファ – slightly more neutral, descriptive.
小さなソファ – often has a slightly more literary, soft, or subjective feel (like “a little sofa” in a somewhat more expressive tone).

In everyday speech, 小さいソファ is perfectly fine and common. Many native speakers wouldn’t feel a big difference in casual contexts.

Why is ソファ written in katakana?

ソファ is a loanword (外来語, がいらいご) from English sofa, so it is written in katakana, which is normally used for:

Loanwords from other languages (テレビ, コーヒー, コンピューター, etc.)
Foreign names
Some onomatopoeia
Certain technical or brand terms

You might also see ソファー with a long vowel mark ー. Both ソファ and ソファー are used.

If you wanted a purely Japanese term, you could say something like ソファー vs 長いす (ながいす) depending on context, but ソファ/ソファー is the standard modern word.

Why is 置きます used instead of 置く? What’s the difference?

置く is the plain (dictionary) form of the verb.
置きます is the polite -ます form.

Use:

置く – casual, used with friends, family, in dictionaries, inner monologue, etc.
置きます – polite, used in normal polite conversation, speaking to customers, teachers, strangers, etc.

So:

壁ぎわに小さいソファを置く。 – casual
壁ぎわに小さいソファを置きます。 – polite

The meaning (to place/put) is the same; only the politeness level differs.

Is ぎわ a separate word? Can I say 壁のぎわ? And why is it sometimes written 際?

Yes, ぎわ (際) is a noun meaning edge / brink / side / margin.

As a standalone word:
- 川ぎわ / 川の際 = riverbank, edge of a river
- 道ぎわ = edge of a road

In compound nouns like 壁ぎわ, it’s usually written together:

壁ぎわ / 壁際 (かべぎわ) = by the wall, at the wall’s edge

You could technically say 壁のぎわ, but in practice 壁ぎわ (sometimes written as 壁際) is the natural compound form, so 壁のぎわ sounds less usual.

About the writing:

ぎわ (hiragana) – neutral, common
際 (kanji) – more formal or in written text: 壁際

Is the word order fixed? Could I say 小さいソファを壁ぎわに置きます instead?

Japanese word order is fairly flexible, as long as the particles stay attached to the right words and the verb comes at the end.

All of these are grammatically fine:

壁ぎわに小さいソファを置きます。
小さいソファを壁ぎわに置きます。

The second one (小さいソファを壁ぎわに置きます) is probably slightly more common, since it introduces the object first: “As for the small sofa, I’ll put it by the wall.”

Changing the order can slightly shift what feels emphasized or what is assumed to be known information, but in simple sentences like this, both are very natural.

How close to the wall does 壁ぎわに imply? Does it mean right against the wall?

壁ぎわに generally means very close to the wall, along the wall’s edge. In many contexts it does imply right up against the wall, as opposed to being somewhere in the middle of the room.

However, it doesn’t absolutely guarantee “physically touching the wall”; it’s more about the area along the wall. If you wanted to stress something like “flush against the wall, with no gap,” you might add more detail (for example, 壁にぴったりつけて), but 壁ぎわに already suggests it’s not placed far out in the room.

In casual speech, can I drop particles like を or に here?

In very casual spoken Japanese, を is sometimes dropped:

小さいソファ(を)置く。

Listeners will still understand that 小さいソファ is the object.

However:

Dropping に is less common, because に is important for showing the location or target.
In addition, your original sentence uses 置きます, which is polite; with polite speech, particles are usually not omitted.

So for natural, polite Japanese, you should keep both:

壁ぎわに小さいソファを置きます。