oiga sinseonhaeseo saelleodeue neoheosseoyo.

가 is the subject marker.

In this sentence, 오이가 신선해서 means the cucumber is/was fresh, so....
The particle 가 marks 오이 as the thing that is being described as fresh.

So:

• 오이 = cucumber
• 오이가 = the cucumber / cucumber + subject marker

Yes, but the nuance changes.

• 오이가 신선해서...: focuses on the cucumber as the subject of being fresh
• 오이는 신선해서...: makes cucumber the topic, often with a slight feeling of as for the cucumber...

So 오이는 can sound more contrastive, like:

• 오이는 신선해서 샐러드에 넣었어요.
  As for the cucumber, it was fresh, so I put it in the salad.

This might imply that some other ingredient was not fresh, or that you are talking specifically about cucumber among several items.

In Korean grammar, 신선하다 is usually treated as a descriptive verb (often called an adjective in learner-friendly explanations).

It works like a verb when it conjugates:

• dictionary form: 신선하다 = to be fresh
• connected form: 신선해서
• polite form: 신선해요

So even though it means an English adjective like fresh, it behaves grammatically more like a Korean verb.

It comes from 신선하다 + -아서/어서.

The steps are:

• 신선하다 = to be fresh
• remove 다 → 신선하-
• with 하다 words, this becomes 해서

So:

• 신선하다 → 신선해서

This means something like:

• being fresh, ...
• because it was fresh, ...
• so it was fresh and...

In this sentence, it gives the reason for the next action.

Here, -아서/어서 connects two clauses and shows a reason or cause.

So:

• 오이가 신선해서 샐러드에 넣었어요.
  = Because the cucumber was fresh, I put it in the salad.
  = The cucumber was fresh, so I put it in the salad.

A good simple way to think of -아서/어서 here is:

• because
• so

It links the cucumber being fresh to the action of putting it in the salad.

This is very common in Korean.

The final verb, 넣었어요, carries the main tense of the sentence. The earlier clause often does not need its tense marked explicitly if the meaning is already clear from context.

So 신선해서 is understood as:

• because it was fresh
• or because it is fresh, depending on context

Since 넣었어요 is past tense, the whole sentence is naturally understood as a past situation: the cucumber was fresh at that time, so it was put into the salad.

Because 에 marks the place/destination that something goes into.

With 넣다 (to put in), Korean often marks the container or destination with 에:

• 상자에 넣다 = put into a box
• 가방에 넣다 = put into a bag
• 샐러드에 넣다 = put into the salad

So 샐러드에 means into the salad.

If you used 샐러드를, that would mark salad as the direct object, which would not fit this sentence the same way.

It is omitted because it is already obvious.

Korean often leaves out words that can be understood from context. In this sentence, the listener already knows the thing being talked about is the cucumber, so there is no need to repeat it.

A fuller version would be something like:

• 오이가 신선해서 샐러드에 오이를 넣었어요.

But this sounds repetitive. Korean usually prefers the shorter, more natural version:

• 오이가 신선해서 샐러드에 넣었어요.

So yes, the object is understood as the cucumber.

Yes.

In the first part:

• 오이가 신선해서 = the cucumber was fresh

Here, 오이 is the subject.

In the second part:

• 샐러드에 넣었어요 = (I) put (it) into the salad

Here, the object is omitted, but it is understood to be the cucumber.

So the same noun is doing two jobs across the sentence:

• subject of being fresh
• understood object of putting into the salad

That is very natural in Korean.

It is omitted.

Korean very often drops pronouns like I, you, he, or she when they are clear from context.

So:

• 샐러드에 넣었어요 literally means something like put into the salad
• but naturally it is understood as I put it into the salad

Because the ending -었어요 is polite and neutral, it does not itself say I. The listener figures that out from the situation.

넣었어요 comes from the verb 넣다 (to put in) and is in the past polite form.

Breakdown:

• 넣다 = to put in
• 넣었어요 = put in / I put in

The ending -어요 makes it polite, and -었- marks past tense.

This is a very common standard polite style used in everyday conversation. It is polite without being especially formal.

Yes, and that sentence is also very natural.

The difference is:

• 오이가 신선해서 샐러드에 넣었어요.
  = Because the cucumber was fresh, I put it in the salad.
  This emphasizes the reason.

• 신선한 오이를 샐러드에 넣었어요.
  = I put fresh cucumber in the salad.
  This simply describes the cucumber as fresh.

So the original sentence is about cause/reason, while 신선한 오이를... is more about describing the noun.

By itself, 오이 does not clearly mark singular or plural. Korean often leaves number unspecified unless it is important.

So depending on context, it could mean:

• a cucumber
• some cucumber
• cucumber as an ingredient

In this sentence, the exact number is probably not important. The meaning is just that cucumber was fresh, so it was added to the salad.

If the speaker really needed to make plurality clear, they could do so in other ways, but normally Korean does not force you to specify it.