fánghào xiě zài dēngjìdān shàng.

A simple way to break it up is:

• 房号 = room number
• 写在 = write on / be written on
• 登记单上 = on the registration form

So the core idea is that the room number and the registration form are linked by 写在, meaning the room number is written on the form, or should be written on the form.

Because Chinese often leaves out words that are understood from context, this short sentence can feel more compact than an English sentence.

It can be understood either way, depending on context.

• In an instruction, form, or hotel-related notice, it can mean: Write the room number on the registration form.
• In ordinary description, it can mean: The room number is written on the registration form.

Chinese often allows this kind of ambiguity when the context is clear.

If you wanted to make the instruction more explicit, you could say:

• 请把房号写在登记单上。

If you wanted a clearly descriptive sentence, context usually does that job, though something like 房号已经写在登记单上了 would sound more clearly like a statement.

Good question. 在 can appear in more than one pattern.

In this sentence, 写在 + place means something like write on or be written on, with emphasis on the location where the writing ends up.

So:

• 写在登记单上 = written on the registration form

Compare this with another common pattern:

• 在登记单上写房号

That version places the location before the verb and means write the room number on the registration form.

Both are natural, but they feel slightly different:

• 写在登记单上 highlights the result/location
• 在登记单上写房号 highlights the action happening at that location

上 is a location suffix meaning on or on the surface of.

So:

• 登记单 = registration form
• 登记单上 = on the registration form

This is very common in Chinese:

• 桌子上 = on the table
• 纸上 = on the paper
• 墙上 = on the wall

Here, 上 is needed because Chinese usually marks this kind of surface location explicitly.

Usually no. 写在登记单 sounds incomplete.

You normally need a location word such as:

• 上 = on
• 里 / 内 = in, inside

So 登记单上 is the natural form for on the registration form.

把 is very common in instructions like this:

• 把房号写在登记单上。

That version is very clear and explicit: Write the room number on the registration form.

The version without 把 is shorter and more compact. Chinese often uses this kind of shortened style in instructions, labels, notes, or situations where the meaning is obvious.

So:

• 把房号写在登记单上 = fuller, clearer instruction
• 房号写在登记单上 = shorter, more compact; can also sound descriptive depending on context

Yes, absolutely. That is very natural.

These are all possible, with slightly different emphasis:

• 在登记单上写房号。 = Write the room number on the registration form.
• 把房号写在登记单上。 = Write the room number on the registration form.
• 房号写在登记单上。 = shorter; can be an instruction or a description depending on context.

A learner will often hear all three patterns, so it is useful to get comfortable with them.

They are very close in meaning.

• 房号 = room number
• 房间号 = room number

房号 is shorter and very common in places like hotels, apartment buildings, or office buildings.
房间号 is a little more explicit because it includes 房间, meaning room.

In many situations, they can be used interchangeably.

登记单 means registration form or registration slip.

It is made up of:

• 登记 = to register
• 单 = form, slip, sheet, list

So 登记单 is literally a registration sheet/form.

Also note that 单 here is pronounced dān.

Yes. The pronunciation is:

• 房号 = fánghào
• 写 = xiě
• 在 = zài
• 登记单 = dēngjìdān
• 上 = shàng

A few useful notes:

• 房 is 2nd tone
• 号 is 4th tone
• 写 is 3rd tone
• 在 is 4th tone
• 登 is 1st tone
• 记 is 4th tone
• 单 is 1st tone
• 上 is 4th tone

There is no major special tone change here, though in natural speech 写 may sound a little lighter before 在. Still, its dictionary tone is 3rd tone.