ki ni su ga aru.

It is read:

き に す が ある。
Ki ni su ga aru.

• 木 = き (ki) = tree
• 巣 = す (su) = nest
• ある = aru = to exist, to be located

In this sentence, に marks the location where something exists.

So:

• 木に = in/on the tree or at the tree, depending on context

With verbs like ある and いる, に often shows the place where something is.

Compare:

• 机に本がある。 = There is a book on the desk.
• 公園に子どもがいる。 = There are children in the park.

が marks the thing that exists: the nest.

So 巣がある literally means:

• a nest exists
• or more naturally, there is a nest

In existence sentences, が is very common because it introduces or identifies what is there.

So:

• 木に巣がある。 = There is a nest in/on the tree.

If you used は, the nuance would change. For example:

• 巣は木にある。 = As for the nest, it is in the tree.

That sounds more like you are talking about the nest as an already known topic.

Because 巣 is not a living being.

In Japanese:

• ある is used for non-living things and abstract things
• いる is used for living things that are animate, especially people and animals

So:

• 木に巣がある。 = There is a nest in the tree.
• 木に鳥がいる。 = There is a bird in the tree.

A nest is an object, so ある is correct.

It can cover either idea in English, depending on the situation.

Japanese often does not force the same distinction English does between in and on.
So 木に巣がある can naturally mean:

• There is a nest in the tree
• There is a nest on the tree

In real-world English, in the tree is usually the most natural translation for a bird’s nest.

Japanese does not normally use articles like a, an, or the.

So 巣 by itself can mean:

• a nest
• the nest
• nests in some contexts

The exact meaning depends on context.

That means 木に巣がある could be understood as:

• There is a nest in the tree.
• The nest is in the tree.

But without context, There is a nest in the tree is the most natural interpretation.

By itself, 木 does not explicitly show singular or plural.

So it could mean:

• tree
• trees

But in this sentence, the most natural reading is usually a tree or the tree, because the sentence sounds like it is describing one location.

Japanese often leaves number unspecified unless it matters.

Japanese and English organize sentences differently.

English:

• There is a nest in the tree.

Japanese:

• 木に = in the tree
• 巣が = nest + subject marker
• ある = exists

So Japanese often builds this kind of sentence as:

[place] + に + [thing] + が + ある / いる

This pattern is very common for saying that something exists somewhere.

Examples:

• 部屋に猫がいる。 = There is a cat in the room.
• 机の上にペンがある。 = There is a pen on the desk.

Yes. 木の上に巣がある is also natural, and it is more specific.

• 木に巣がある = There is a nest in/on the tree.
• 木の上に巣がある = There is a nest on top of / up in the tree.

木に is simpler and more general.
木の上に emphasizes the nest’s position as being up on the tree.

Depending on context, both can work.

Not in this sentence.

• に marks the place where something exists
• で marks the place where an action happens

So:

• 木に巣がある。 = There is a nest in the tree. ✅
• 木で巣がある。 = unnatural / incorrect in this meaning ❌

But で would be correct in a sentence like:

• 鳥が木で休んでいる。 = A bird is resting in the tree.

Here, resting is an action/state occurring there, so で can be used depending on the verb and phrasing.

Yes, it can, depending on context.

Japanese often leaves that open.

So 木に巣がある might mean:

• There is a nest in the tree.
  a specific observation

or

• Trees have nests in them / A tree has a nest in it.
  in a descriptive or explanatory context

Still, the most straightforward reading is usually a specific existence statement: There is a nest in the tree.