oiga sinseonhaeseo saelleodeue neoheosseoyo.

The cucumber was fresh, so I put it in the salad.

Breakdown of oiga sinseonhaeseo saelleodeue neoheosseoyo.

• ~가: subject particle
• ~에: location particle
• 넣다: to put
• 샐러드: salad

Questions & Answers about oiga sinseonhaeseo saelleodeue neoheosseoyo.

What does 가 in 오이가 do?

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

Could I use 오이는 instead of 오이가?

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.

Is 신선하다 a verb or an adjective?