oneureun muryo baesong kuponi isseoseo sinbareul deo ssage sasseo.

오늘은 uses the topic particle 은. It gives a sense of as for today or today, ... and sets the time frame for the whole sentence.

• 오늘 신발을 샀어 = I bought shoes today.
• 오늘은 신발을 샀어 = As for today / today, I bought shoes.

The 은 can also add a slight contrastive feeling, depending on context, such as today at least or today, unlike other days.

Because 오늘 is not the subject here, and this sentence is not using today as a location/time point marked by 에.

• 오늘이 would make today the subject, which does not fit this sentence naturally.
• 오늘에 is usually not used for a general today meaning like this.
• 오늘은 is the natural way to say as for today / today as the topic.

Korean often stacks nouns before the main noun.

• 무료 = free
• 배송 = delivery, shipping
• 쿠폰 = coupon

So 무료 배송 쿠폰 literally means free-delivery coupon, or more naturally in English, a free shipping coupon.

The last noun, 쿠폰, is the main noun, and the words before it describe it.

Because 있다 uses 이/가, not 을/를.

• 쿠폰이 있다 = there is a coupon / I have a coupon
• 쿠폰을 있다 = incorrect

In Korean, 있다 marks the thing that exists with 이/가. So 무료 배송 쿠폰이 있어서 means because there was a free shipping coupon or because I had a free shipping coupon.

Here it means because or since.

The pattern -아서/어서 often connects two clauses and can show reason:

• 쿠폰이 있어서 신발을 샀어 = Because there was a coupon, I bought shoes.

Sometimes -아서/어서 can just connect events, but in this sentence the most natural reading is clearly reason/cause.

In this kind of sentence, 있어서 is the normal form.

Korean often does not mark past separately in a reason clause with -아서/어서 the way English does. The past meaning is already clear from the final verb 샀어.

So:

• 무료 배송 쿠폰이 있어서 신발을 샀어 = Because there was / I had a free shipping coupon, I bought shoes.

For 있다, 있어서 is the natural form here. Learners often expect a past-marked form, but Korean does not usually say it that way in this structure.

싸다 is an adjective meaning to be cheap.
Adding -게 changes it into an adverb:

• 싸다 = to be cheap
• 싸게 = cheaply / at a cheaper price

So 더 싸게 샀어 means bought it more cheaply or got it for cheaper.

Here, 더 means more.

That comparison is implied.

더 싸게 usually means:

• cheaper than usual
• cheaper than the original price
• cheaper than without the coupon
• cheaper than expected

Korean often leaves the comparison unstated if it is obvious from context. In this sentence, the free shipping coupon is the reason the total purchase felt cheaper.

신발 can mean shoes, a pair of shoes, or sometimes just footwear, depending on context.

Korean nouns usually do not need:

• a / the
• singular/plural marking

So 신발을 샀어 can naturally mean I bought shoes.

The particle 을 marks 신발 as the object of 샀어.

The base form is 사다, meaning to buy.

• 사다 = to buy
• 샀어 = bought

샀어 is casual, informal speech. You would use it with friends, people younger than you, or in very relaxed conversation.

More polite versions would be:

• 샀어요 = polite casual
• 샀습니다 = formal polite

The subject is omitted because Korean often leaves it out when it is obvious from context.

So this sentence naturally means:

• I had a free shipping coupon, so I bought shoes more cheaply.

But depending on context, it could also mean he/she/they if that has already been established. Korean does not need to repeat the subject as often as English does.

Yes, that is possible.

• 오늘 무료 배송 쿠폰이 있어서 신발을 더 싸게 샀어
• 오늘은 무료 배송 쿠폰이 있어서 신발을 더 싸게 샀어

Both are natural. The version with 은 sounds a little more topical and can suggest a slight contrast or emphasis on today. Without 은, it is a bit more neutral and straightforward.