kamo ga suimen ni iru.

A common reading is:

鴨 = かも
が = が
水面 = すいめん or みなも
に = に
いる = いる

So the whole sentence could be read as either:

かも が すいめん に いる
or
かも が みなも に いる

A learner often wonders about 水面 because it has more than one reading. すいめん is more neutral and dictionary-like, while みなも feels more native and sometimes more literary. If your textbook gives furigana, follow that.

が marks 鴨 as the grammatical subject of the sentence.

So the structure is basically:

鴨 が = the duck is the subject
水面 に = at/on the water’s surface
いる = exists / is

In this kind of sentence, が often presents something as new information or identifies what is in that location.

Both are possible, but they create different nuances.

鴨が水面にいる。
This sounds like you are pointing out or identifying the duck: There is a duck on the water or A duck is on the surface of the water.

鴨は水面にいる。
This makes 鴨 the topic: As for the duck, it is on the water’s surface. This often sounds like the duck is already known in the conversation, or it may create contrast with something else.

So が is very natural when you are simply noticing or introducing the duck.

With いる and ある, に marks the place where something exists.

So:

水面にいる = to be at/on the water’s surface

This is a very common pattern in Japanese:

場所 + に + いる for animate things
場所 + に + ある for inanimate things

Even though に is often taught as meaning to, here it is not showing direction. It is showing location.

Because a duck is animate.

Japanese usually uses:

• いる for people and animals
• ある for things and objects

Since 鴨 is an animal, いる is the correct choice.

Because で usually marks the place where an action happens, while に marks the place where something exists.

Here, いる is an existence verb, so に is the normal particle.

Compare:

鴨が水面にいる。
The duck is on the water.

鴨が池で泳いでいる。
The duck is swimming in the pond.

In the second sentence, 泳いでいる describes an action, so で is used for the location of that action.

Yes. In English, we would usually say on the water or on the surface of the water, but Japanese does not always match English prepositions one-for-one.

With いる, に simply marks the location where the duck is. Since 水面 means the surface of the water, the natural English translation is on the water’s surface or on the water.

So this is a good example of why you should not translate particles too literally.

It mainly says that the duck is present there. The focus is on existence or location, not on the manner of movement.

If you want to say the duck is specifically floating, Japanese would more clearly say something like:

鴨が水に浮いている。
or
鴨が水面に浮かんでいる。

So いる tells you the duck is there, while 浮いている / 浮かんでいる tells you it is floating.

Yes. Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

For example:

鴨が水面にいる。
水面に鴨がいる。

Both are natural. The basic meaning stays the same, but the focus shifts a little.

• 鴨が水面にいる starts with the duck
• 水面に鴨がいる starts with the location and then tells you what is there

Japanese often puts the location earlier when setting the scene.

Japanese does not have articles like English a, an, and the.

So 鴨 can mean:

• a duck
• the duck
• sometimes just duck(s) in a general sense

The exact meaning depends on context. That is normal in Japanese, and learners have to get used to understanding definiteness from the situation rather than from a separate word.

Yes, 水面 is correct, but it is a bit more specific than a simpler expression.

水面 means the surface of the water. So the sentence focuses on the duck being on that surface.

In everyday speech, people might also say something like:

鴨が水の上にいる。
The duck is on the water.

That version may feel more straightforward to a beginner.
So 水面 is not wrong at all, but it is a slightly more exact vocabulary choice.

鴨 often refers to a duck, and it can especially suggest a wild duck.

If you mean a domestic duck, Japanese often uses アヒル.

That said, in simple sentences and basic learning materials, 鴨 is often just understood as duck unless the distinction matters.