isu no sita de yubiwa wo mituketa.

It is read:

いす の した で ゆびわ を みつけた。

Word by word:

• 椅子（いす） = chair
• の = linking particle, often like of
• 下（した） = under / below
• で = at / in / on / under, marking the place where an action happens
• 指輪（ゆびわ） = ring
• を = marks the direct object
• 見つけた（みつけた） = found

の connects 椅子 and 下.

So:

• 椅子 = chair
• 下 = under / below
• 椅子の下 = under the chair or literally the chair’s underside/below the chair

This is a very common pattern in Japanese:

• 机の上 = on the desk
• 家の中 = inside the house
• 学校の前 = in front of the school

So X の 下 means under X.

Because で marks the place where the action happens.

Here, the action is 見つけた = found.
The finding happened under the chair, so Japanese uses で:

• 椅子の下で指輪を見つけた。
• I found a ring under the chair.

A useful way to think of it:

• で = where an action takes place
• に = where something exists, goes, or ends up

Compare:

• 椅子の下で指輪を見つけた。 = I found a ring under the chair.
  
  • finding is an action, so で
• 椅子の下に指輪がある。 = There is a ring under the chair.
  
  • existence, so に

を marks the direct object of the verb.

The verb is 見つけた = found.
What was found? 指輪.

So:

• 指輪を見つけた = found a ring

This is a very standard pattern:

• 本を読んだ = read a book
• 映画を見た = watched a movie
• 鍵を見つけた = found the key

Japanese normally puts the verb at the end of the sentence.

So instead of English order:

• I found a ring under the chair

Japanese is more like:

• Under the chair, ring, found

That gives:

• 椅子の下で 指輪を 見つけた。

This is one of the biggest word-order differences between English and Japanese. In very general terms, Japanese is a subject/object/verb language, while English is subject/verb/object.

Japanese often leaves out the subject when it is understood from context.

So 椅子の下で指輪を見つけた。 can naturally mean:

• I found a ring under the chair
• We found a ring under the chair
• sometimes even He/She found a ring under the chair

The exact subject depends on the situation.

In English, you usually need to say the subject. In Japanese, if it is obvious, it is often omitted.

If you wanted to include it, you could say:

• 私は椅子の下で指輪を見つけた。 = I found a ring under the chair.

But very often, 私は is unnecessary.

見つけた is the plain past form of 見つける.

• dictionary form: 見つける = to find
• plain past: 見つけた = found
• polite past: 見つけました = found

So this sentence is in a casual/plain style:

• 椅子の下で指輪を見つけた。

A polite version would be:

• 椅子の下で指輪を見つけました。

Not exactly.

• 見つけた means someone found it
• 見つかった means it was found / turned up

So:

• 椅子の下で指輪を見つけた。 = I found a ring under the chair.
• 椅子の下で指輪が見つかった。 = A ring was found under the chair.

The first focuses more on the finder.
The second focuses more on the ring being discovered.

Yes. 椅子の下 is a single location phrase meaning under the chair.

Then で attaches to that whole phrase:

• 椅子の下 = under the chair
• 椅子の下で = under the chair, as the place where the action happened

So it is best to group it like this:

• [椅子の下で] [指輪を] [見つけた]

That helps you see the sentence structure clearly.

Japanese normally does not use spaces between words in ordinary writing.

So the normal written sentence is:

椅子の下で指輪を見つけた。

Spaces are often added in teaching materials to help beginners see the parts more clearly. They are just a learning aid.

Yes, in this sentence 椅子の下 means the area beneath or under the chair.

In most everyday situations, under the chair is the most natural English translation.

Japanese 下 is broad and can mean:

• under
• below
• beneath
• lower part

But in this sentence, under is the best fit.

Just change the verb to the polite past form:

• 椅子の下で指輪を見つけました。

Everything else stays the same.

So:

• plain/casual: 見つけた
• polite: 見つけました

Good question. They are related, but not the same.

• 見つけた = found / discovered
• 拾った（ひろった） = picked up

So:

• 椅子の下で指輪を見つけた。
  = I found a ring under the chair.

  • You discovered it there.

• 椅子の下で指輪を拾った。
  = I picked up a ring from under the chair.

  • You physically picked it up.

If you want to focus on noticing or discovering it, 見つけた is correct.
If you want to focus on taking it in your hand, 拾った is better.