uekibati wo mizuukezara ni oku.

It is read:

うえきばち を みずうけざら に おく。

Word by word:

• 植木鉢 = うえきばち
• 水受け皿 = みずうけざら
• 置く = おく

A useful detail: 受け皿 is read うけざら, not うけさら. The さ becomes ざ here.

No. In normal Japanese, this would usually be written without spaces:

植木鉢を水受け皿に置く。

Spaces are often added in beginner materials to make the particles and word boundaries easier to see.

Because 植木鉢 is the direct object of 置く.

In other words, it is the thing being moved or placed.

So in:

植木鉢を水受け皿に置く

the flowerpot is the thing you are placing, so it gets を.

Here, に marks the destination or target location of the action.

With verbs like 置く, 入れる, 座る, and 行く, に often shows where something ends up.

So:

• 植木鉢を = the thing being placed
• 水受け皿に = the place it is being placed onto / into / at

Not exactly by itself. Japanese particles do not match English prepositions one-for-one.

Here, に marks the target location, and in English that target is naturally translated as on the saucer/tray.

So に is not literally the word on, but in this sentence that is the most natural English translation.

Because で and に do different jobs.

• に = destination, endpoint, result location
• で = place where an action happens

With 置く, the important thing is where the object ends up, so に is used.

Compare:

• 机の上に本を置く。 = Put the book on the desk.
• 机の上で本を読む。 = Read a book on the desk / at the desk.

In the first sentence, the desk is the destination of placement, so に is correct.

Yes, absolutely.

• 水受け皿に置く = put it on the drip tray / saucer
• 水受け皿の上に置く = put it on top of the drip tray / saucer

The version with の上に is a little more explicit about physical top.
The shorter 水受け皿に置く is still natural and common.

The dictionary form is often used in Japanese for:

• simple statement style
• instructions
• labels
• neutral written descriptions
• sentence fragments in exercises

So this can feel like a plain instruction or an example sentence.

Depending on context, you might also see:

• 植木鉢を水受け皿に置きます。 = polite statement
• 植木鉢を水受け皿に置いてください。 = please put the flowerpot on the saucer
• 植木鉢を水受け皿に置け。 = strong command

Because Japanese often omits the subject when it is obvious or not important.

This sentence does not explicitly say who is doing the action. Depending on context, it could mean things like:

• Put the flowerpot on the saucer.
• (Someone) puts the flowerpot on the saucer.
• To place the flowerpot on the saucer.

Japanese very often leaves out I, you, he, she, etc., if the listener can infer them.

Not completely.

Because the particles show each word’s role, Japanese word order is more flexible than English. You could also say:

水受け皿に植木鉢を置く。

This still means the same thing.

That said, the verb usually comes at the end, and some orders feel more natural depending on emphasis.

Yes, it is a compound noun.

It is made from:

• 水 = water
• 受け = receiving, catching
• 皿 = dish, plate

So the literal idea is something like water-catching dish, which matches the idea of a saucer or drip tray placed under a plant pot.

Yes, but the nuance changes a little.

• 置く = put/place; a very general and natural verb
• 載せる = place something onto a surface; can emphasize setting it on top
• 入れる = put into; not a good fit unless you mean putting something inside a container

So 植木鉢を水受け皿に置く is a very safe, natural choice.

If you said 植木鉢を水受け皿に載せる, that could also work, but it sounds a bit more specifically like placing it onto the surface.