Learn Hardcore Japanese: watasi ha heya no kabe ni posutaa wo harimasu.

に
- Target / destination / final position
- 壁にポスターを貼ります。
  → I put a poster on the wall (the poster ends up on the wall).
で
- Place where an action happens
- 部屋でポスターを貼ります。
  → I put up posters in my room (the room is the place where I do the action; it doesn’t say where I stick them).
へ
- Direction toward something (similar to “to/toward”)
- Not natural here, because you aren’t moving toward the wall; you’re attaching something to it.

So 壁に is correct because the wall is the surface the poster ends up on.

What does を do in ポスターを貼ります?

を marks the direct object of the verb — the thing that the action is done to.

ポスター = poster
ポスターを貼ります = (I) stick / put up a poster

Structure:

[direct object] + を + [verb]
- 本を読みます。= (I) read a book.
- 音楽を聞きます。= (I) listen to music.
- ポスターを貼ります。= (I) put up a poster.

So in this sentence, ポスター is the thing being 貼る’d, and を marks it.

What exactly does the verb 貼ります mean, and what is its dictionary form?

The verb is:

Dictionary form: 貼る
Polite non-past form: 貼ります

Meaning: to stick / to paste / to put up (by sticking), usually something flat to a surface:

ポスターを壁に貼る = stick a poster on the wall
切手を封筒に貼る = put a stamp on an envelope
メモをドアに貼る = stick a note on the door

The polite non-past 貼ります can mean:

a future action: “(I) will put up a poster.”
a habitual action: “(I) put up posters.”

Japanese non-past form covers both present and future, so context decides which.

Does this sentence mean “I will put up a poster” or “I put up a poster (habitually)”? How do I know?

私は部屋の壁にポスターを貼ります。 by itself is ambiguous: grammatically, it can be:

Future / planned: “I will put a poster on the wall.”
Habitual: “I (usually) put posters on the wall.”
General fact: “I put a poster on the wall” (describing a procedure, etc.).

Which one it is depends on context or extra words, for example:

明日、部屋の壁にポスターを貼ります。
→ Tomorrow I will put a poster on the wall. (clearly future)
毎年、新しいポスターを部屋の壁に貼ります。
→ Every year I put a new poster on the wall. (habit)

Japanese often leaves tense/aspect to context instead of marking it explicitly like English.

Can I leave out 私 and just say 部屋の壁にポスターを貼ります?

Yes, and that’s actually more natural in many conversations.

Japanese often omits the subject when it’s clear from context. If it’s obvious you’re talking about yourself, you would typically say:

部屋の壁にポスターを貼ります。
→ (I) will put a poster on the wall.

You usually include 私 when:

You want to be very clear it’s you (e.g., contrast with someone else).
In self-introductions or formal writing.

In casual spoken Japanese, 私は is often left out unless there’s a reason to stress “I.”

Can I change the word order, like ポスターを部屋の壁に貼ります? Is that okay?

Yes, that’s fine. Japanese word order is fairly flexible as long as:

The verb comes at the end, and
The particles stay attached to the right words.

All of these are acceptable and natural:

私は部屋の壁にポスターを貼ります。
私はポスターを部屋の壁に貼ります。
部屋の壁にポスターを貼ります。
ポスターを部屋の壁に貼ります。

The meaning is basically the same. Slight differences in emphasis can appear (for example, putting ポスターを earlier can slightly highlight “a poster”), but for learners, you can treat them as equivalent.

Why does the Japanese not say “my room”? How do I know it’s my room?

部屋 by itself just means “a/the room.” There is no explicit “my” in the sentence.

If the context is clearly about your room, Japanese usually omits the possessive. It’s understood from context:

In English: “I will put a poster on the wall of my room.”
In Japanese: (私の)部屋の壁にポスターを貼ります。
→ The 私の is usually dropped if it’s obvious.

If you need to be explicit, you can say:

私の部屋の壁にポスターを貼ります。
= I will put a poster on the wall of my room.

But very often, 私の is unnecessary and sounds a bit heavy if the ownership is obvious.

How do I say “I will put posters (plural) on the wall”? Does ポスター need a plural ending?

Japanese usually does not mark plural for inanimate nouns like “poster.”

ポスター can mean:
- a poster
- posters depending on context.

Your sentence:

私は部屋の壁にポスターを貼ります。

can be translated as:

“I will put a poster on the wall.”
“I will put posters on the wall.”

If you really need to emphasize there are many posters, you can add extra words:

たくさんポスターを貼ります。 = I will put up many posters.
何枚かポスターを貼ります。 = I will put up a few posters.

What’s the difference between 貼る, かける, and 置く when talking about putting things in a room?

These verbs describe different ways of placing something:

貼る（貼ります）
- To stick / paste something to a surface
- Used for posters, stickers, stamps, notes, etc.
- ポスターを壁に貼る = stick a poster on the wall.
かける（かけます）
- To hang something (often from a hook, nail, hanger, etc.)
- カレンダーを壁にかける = hang a calendar on the wall.
- コートをハンガーにかける = hang a coat on a hanger.
置く（置きます）
- To place / put / set down something (usually resting on a surface, not attached)
- 本を机の上に置く = put a book on the desk.
- 花を棚に置く = put flowers on the shelf.

So you use 貼る specifically because a poster is attached flat to the wall.

How would this sentence look in casual, non-polite Japanese?

In casual speech, you usually use the dictionary form instead of -ます form.

Polite:

私は部屋の壁にポスターを貼ります。

Casual versions (all natural):

部屋の壁にポスター貼る。
部屋の壁にポスターを貼る。 (with を kept in)
私、部屋の壁にポスター貼る。 (if you want to mention “I”)

Common changes in casual speech:

Drop 私 if it’s obvious.
Sometimes drop を in conversation (especially before certain verbs), though keeping it is also fine and clear.
Use the plain form 貼る instead of 貼ります.