yangmareul an sineossdeoni sae sinbari jogeum apasseo.

In Korean, 신다 is the general verb for put on / wear things on your feet. That includes:

• 신발을 신다 = to wear shoes
• 양말을 신다 = to wear socks

So 양말을 안 신었더니 literally means because I didn’t wear socks or when I didn’t wear socks.

-었더니 / -았더니 / -더니 often connects a first action or situation to a result that followed.

Here, 양말을 안 신었더니 새 신발이 조금 아팠어 means something like:

• I didn’t wear socks, so my new shoes hurt a little
• When I didn’t wear socks, my new shoes hurt a little

This ending often gives a feeling of:

• a cause leading to a result, or
• the speaker noticing the result afterward

So it is not just simple sequence. It suggests not wearing socks led to the shoes hurting.

Because Korean uses different verbs depending on the type of clothing:

• 입다 = wear clothes on the body
• 쓰다 = wear something on the head
• 끼다 = wear something on fingers/hands, or insert
• 차다 = wear something like a watch or accessory
• 신다 = wear things on the feet

Since socks go on your feet, Korean says 양말을 신다, not 양말을 입다.

Because 양말 is the object of 신다.

• 양말을 신다 = to wear socks
• 양말을 안 신다 = to not wear socks

The particle 을/를 marks what is being worn.

So:

• 양말을 = socks as the object
• 안 신었더니 = didn’t wear, and then/as a result

This is very natural in Korean. Korean often says that the thing causing discomfort is what hurts.

So 새 신발이 아팠어 literally looks like:

• the new shoes hurt

In natural English, you might say:

• My new shoes hurt
• The new shoes were uncomfortable
• The new shoes hurt my feet

Korean does not need to explicitly mention my feet here if the meaning is obvious.

It means hurt a little or were a little painful/uncomfortable.

• 조금 = a little
• 아팠어 = hurt / was painful

With shoes, 아프다 often means they caused physical pain or soreness, usually by rubbing or being tight. So the sentence suggests the new shoes were not just mildly inconvenient—they caused some actual discomfort.

Because both parts refer to past events.

• 안 신었더니 = I didn’t wear socks
• 아팠어 = the new shoes hurt

The speaker is talking about a completed situation in the past:

1. they did not wear socks,
2. as a result, the new shoes hurt.

This is very natural in Korean when describing a past cause and its past result.

아팠어 is casual / informal speech.

The dictionary form is 아프다.
The polite version would be:

• 양말을 안 신었더니 새 신발이 조금 아팠어요.

So the original sentence sounds like something you would say to:

• a friend
• someone younger
• someone you are close to

In this sentence, the most natural interpretation is because I didn’t wear socks.

Although -었더니 can sometimes describe a sequence like I did X, and then I noticed Y, here the relationship is clearly causal:

• not wearing socks
• led to the shoes hurting

So an English learner should understand it mainly as a cause-and-result sentence.

It could, but it would mean something slightly different.

• 발이 아팠어 = my feet hurt
• 신발이 아팠어 = the shoes hurt / the shoes were painful

The original sentence focuses on the shoes as the source of the pain, not the feet as the experiencer. Korean often expresses discomfort this way.

So:

• 발이 아팠어 focuses on your body
• 신발이 아팠어 focuses on the shoes causing the problem

Usually 새 신발 means new shoes, and in many contexts it strongly suggests newly bought shoes or shoes being worn for the first time / not broken in yet.

That matters here, because new shoes commonly hurt when worn without socks. So the sentence naturally gives that feeling even without extra explanation.