Learn Hardcore Japanese: watasi ha musume no e wo heya no kabe ni kazarimasu. - I put my daughter's pictures on the wall of the room.

It shows that 私 (I) is the topic of the sentence: as for me / talking about me.
The actual grammatical subject is often marked by が, but in this sentence the subject and topic are effectively the same person, so you just see 私は.
In normal conversation, you would often drop 私は completely if it’s clear from context who is doing the action.

Why is 娘の絵 understood as my daughter's picture? Where is my in Japanese?

Japanese often leaves out possessive words like my, your, his, etc. when they’re obvious from context.

娘 just means daughter in general.
Because the topic is 私 (I), listeners naturally understand 娘 as my daughter unless context says otherwise.
If you needed to make it explicit, you could say 私の娘の絵 (a picture of my daughter / my daughter's picture), but it often sounds unnecessarily long if the relationship is obvious.

Does 娘の絵 mean “a picture of my daughter” or “a picture drawn by my daughter”?

On its own, 娘の絵 is ambiguous; it just means daughter + picture.

Typical readings:

Picture drawn by my daughter
→ “my daughter’s drawing” (the daughter is the artist)
Picture of my daughter
→ “a picture of my daughter” (the daughter is the subject of the picture)

Which one is meant depends on context. If you want to be clear, you can say:

娘が描いた絵 – a picture drawn by my daughter
娘の写真 – a photo of my daughter (for clarity, you’d usually say 写真 rather than 絵)

What is the role of を in 娘の絵を?

を marks the direct object of the verb.

娘の絵を = (I) [directly] act on my daughter's picture / drawing
The verb 飾ります (to decorate / display) is a transitive verb, so the thing being decorated or displayed takes を.
Structure:
私 (topic) + 娘の絵を (object) + …飾ります (decorate).

Why is 部屋の壁 using の? What exactly does 部屋の壁 mean?

の links two nouns and makes an “X of Y / Y’s X” relationship.

部屋 = room
壁 = wall
部屋の壁 = the wall of the room / the room’s wall

So 部屋の壁に is “on/in the wall of the room”.
This is one noun phrase: (部屋の壁), not 部屋 + 壁 separately.

Why is に used after 壁 in 壁に飾ります, and not で?

With verbs like 飾る (to decorate / display / hang), に marks the target location where something ends up.

壁に飾ります = “display (it) on the wall / hang it on the wall.”
に here is “onto / on / at (as a final place).”
で would mark the place where an action happens, like “I work in the room” (部屋で働きます), not the target surface you put something on.

So:

壁に飾る – put/display something on the wall (target)
部屋で飾る – do the decorating in the room (location of action)

There’s no particle after 部屋 in 部屋の壁. Isn’t every noun supposed to have a particle?

Particles are needed for sentence elements, but inside a noun phrase, you don’t use case particles like は, が, を, に.

部屋の壁 is a single noun phrase: “the wall of the room.”
Here, 部屋 is just modifying 壁 via の, so it doesn’t take another particle.
The whole phrase 部屋の壁 then takes に: 部屋の壁に飾ります.

The verb is 飾ります, which is non‑past. How can this sentence mean both “I decorate” and “I will decorate”?

Japanese has basically two main tenses for verbs: non‑past and past.

飾ります (non‑past) can cover:
- a general/habitual action: I decorate / I usually decorate
- a scheduled or future action: I will decorate / I’m going to decorate
Which nuance is correct is decided by context, not by verb form alone.

So 飾ります can be translated as decorate / will decorate, depending on the situation given.

Why is the verb in the -ます form (飾ります) instead of the dictionary form 飾る? When would I use 飾る?

飾ります is the polite form; 飾る is the plain (dictionary) form.

Use 飾ります when speaking politely: to strangers, teachers, customers, etc.
Use 飾る:
- in casual speech with friends/family (e.g. 部屋の壁に飾る)
- in dictionaries, grammar explanations, and many written forms.

The core meaning is the same; only the level of politeness changes.

Can I change the word order? For example, can I say 私は部屋の壁に娘の絵を飾ります or 娘の絵を私は部屋の壁に飾ります?

Yes. Japanese word order is relatively flexible as long as the verb stays at the end and particles are correct.

All of these are grammatical:

私は娘の絵を部屋の壁に飾ります。 (original)
私は部屋の壁に娘の絵を飾ります。
娘の絵を部屋の壁に飾ります。 (dropping 私は)
娘の絵を私は部屋の壁に飾ります。 (emphasizes 娘の絵)

Changing the order changes focus / emphasis a bit, but not the basic meaning.

Can I drop 私は and just say 娘の絵を部屋の壁に飾ります?

Yes, and in natural conversation, you very often would.

If it’s already clear who is speaking and who is doing the action, 私は is usually omitted.
The sentence 娘の絵を部屋の壁に飾ります。 would still be understood as I doing the decorating in most contexts, unless there’s a reason to think it’s someone else.

Does 壁に mean “on the wall” or “to the wall”? How do you say “on the wall” in Japanese?

In this context, 壁に is best translated as on the wall.

With verbs like 飾る, 貼る (to stick), かける (to hang), に marks the surface where something is attached or displayed.
So 壁に飾ります = “(I) decorate / hang it on the wall.”

If you want to emphasize “on top of” a horizontal surface, you would use 上に (e.g. 机の上に置きます – put it on the desk), but for vertical walls, just 壁に is normal.

Are 娘 and 絵 singular or plural here? Is this “my daughter’s picture” or “my daughters’ pictures”?

Japanese nouns usually don’t mark singular/plural, so it depends on context.

Possible readings:

my daughter’s picture / my daughter’s pictures
my daughters’ picture / my daughters’ pictures

The sentence itself doesn’t specify number; you choose picture / pictures in English based on the situation that the learner already knows from context.