eojeneun sogogiboda dwaejigogiga deo ssaseo dwaejigogireul sasseo.

어제는 means something like as for yesterday or yesterday, at least. The 는 makes 어제 the topic and can add a slight contrastive nuance.

So:

• 어제 소고기보다 돼지고기가 더 싸서... = Yesterday, pork was cheaper than beef, so...
• 어제는 소고기보다 돼지고기가 더 싸서... = As for yesterday, pork was cheaper than beef, so...

This can imply a contrast such as maybe on other days it is different, but yesterday this was true.

Also, time words like 어제 often appear with no particle at all, so 는 is not required, but it adds nuance.

This follows a very common Korean comparison pattern:

A보다 B가 더 + adjective

which means:

B is more adjective than A

So here:

• 소고기보다 = than beef
• 돼지고기가 = pork
• 더 싸서 = is cheaper, so...

Put together:

소고기보다 돼지고기가 더 싸서
= Because pork was cheaper than beef...

A useful way to remember it is:

• A보다 = than A
• B가 = B is the thing being described
• 더 = more
• 싸다 = to be cheap

Because in the first clause, 돼지고기 is the thing being described as cheap, so it takes the subject marker 가.

• 돼지고기가 더 싸다 = Pork is cheaper
• 돼지고기를 사다 = to buy pork

So the same noun appears twice in different roles:

1. 돼지고기가 = pork is the subject of 싸다
2. 돼지고기를 = pork is the object of 샀어

This is very normal in Korean.

보다 here is a comparison particle meaning than.

So:

• 소고기보다 = than beef
• 나보다 = than me
• 어제보다 = than yesterday

It is attached directly to the noun being used as the standard of comparison.

Examples:

• 형보다 키가 커요. = I am taller than my older brother.
• 커피보다 차를 더 좋아해요. = I like tea more than coffee.

In your sentence, 소고기보다 sets beef as the thing pork is being compared to.

더 means more, and it commonly appears in comparisons.

So:

• 소고기보다 돼지고기가 더 싸다 = Pork is cheaper than beef

In many cases, Korean speakers can omit 더 if the comparison is already clear from 보다:

• 소고기보다 돼지고기가 싸다

This still means Pork is cheaper than beef.

However, using 더 is very common and often sounds clearer and more natural, especially for learners.

싸서 comes from 싸다 (to be cheap) + -아서/어서.

Here, -서 connects the two parts of the sentence and gives a reason:

• 싸서 = because it was cheap / being cheap, so...

So the sentence structure is:

• 돼지고기가 더 싸서 = because pork was cheaper
• 돼지고기를 샀어 = I bought pork

Together:

Because pork was cheaper than beef, I bought pork.

In this sentence, -서 is working like because / so.

This is a very common learner question.

Even though the whole situation is in the past, Korean usually does not put past tense directly before -서 in this kind of reason clause. The past time is understood from context and from the final verb 샀어.

So:

• 돼지고기가 더 싸서 샀어 = natural
• 돼지고기가 더 쌌어서 샀어 = usually unnatural

Korean often works like this:

• 배가 아파서 집에 갔어. = I went home because my stomach hurt.
• 비가 와서 안 갔어. = I didn’t go because it rained.

Even though these are past events, the first clause often stays in this -아/어서 form.

Korean often repeats nouns where English would use it.

Here:

• 돼지고기가 더 싸서 = because pork was cheaper
• 돼지고기를 샀어 = I bought pork

In English, we would usually say:

Because pork was cheaper than beef, I bought it.

But in Korean, repeating 돼지고기 is very natural and often clearer than using a pronoun.

You could omit the second 돼지고기 if the context is very clear:

• 어제는 소고기보다 돼지고기가 더 싸서 샀어.

This means Yesterday, pork was cheaper than beef, so I bought it.
But the full version with 돼지고기를 is clearer and very natural.

It is omitted because Korean often leaves out subjects when they are obvious from context.

English usually needs a subject:

• I bought pork.

Korean often does not:

• 돼지고기를 샀어.

In your sentence, the person who bought the pork is understood from the situation, so there is no need to say 내가 or 제가 unless you want to emphasize I specifically.

If you did include it, it would be:

• 나는 어제는 소고기보다 돼지고기가 더 싸서 돼지고기를 샀어.

But this would usually sound unnecessary unless you are contrasting yourself with someone else.

샀어 is the casual, non-polite past form of 사다 (to buy).

Formation:

• 사다 → 사았어 → 샀어

This is used with:

• close friends
• younger people
• family
• people you speak casually with

If you want the polite version, you would say:

• 샀어요

So the full polite sentence would be:

어제는 소고기보다 돼지고기가 더 싸서 돼지고기를 샀어요.

Same meaning, just more polite.

It can, but the nuance changes.

• 돼지고기가 더 싸서 is the most neutral way to say pork was cheaper
• 돼지고기는 더 싸서 adds a stronger topic or contrast feeling, something like as for pork, it was cheaper

So:

• 소고기보다 돼지고기가 더 싸서... = neutral comparison
• 소고기보다 돼지고기는 더 싸서... = more contrastive, possibly emphasizing pork as a topic

In this sentence, 가 sounds the most natural because 돼지고기 is simply the thing being described as cheaper.