gotongi simhaesseo.

이 is the subject marker. It marks 고통 as the thing being described.

• 고통이 심했어 = The pain/suffering was severe
• Here, 고통 is the subject, and 심했어 describes its state.

A learner might compare this with 은/는:

• 고통이 심했어 → neutral, straightforward description
• 고통은 심했어 → adds contrast or topic emphasis, like As for the pain, it was severe or The pain, at least, was severe

So 이 is the most natural choice if you are simply stating a fact.

심했어 comes from the dictionary form 심하다.

Breakdown:

• 심하다 = to be severe, serious, intense
• 심했어 = was severe / was intense

This is the past tense casual form.

So:

• 심하다 → present dictionary form
• 심해 → present casual
• 심했어 → past casual

In Korean, words that English speakers often think of as adjectives usually behave more like descriptive verbs.

So 심하다 is often translated as to be severe, not just severe.

That is why it can conjugate for tense:

• 심해 = is severe
• 심했어 = was severe
• 심할 거야 = will be severe

This is completely normal in Korean grammar.

Because the base form is 심하다, not 심다.

When 하다 verbs or adjectives conjugate, they often become:

• 하다 → 해
• past tense: 하다 → 했어

So:

• 심하다 → 심해
• 심하다 → 심했어

This same pattern appears in many other words:

• 이상하다 → 이상해 / 이상했어
• 중요하다 → 중요해 / 중요했어

It is casual/informal speech.

Levels:

• 고통이 심했어 = casual
• 고통이 심했어요 = polite
• 고통이 심했습니다 = formal/polite

You would use 심했어 with friends, people younger than you, or in narration where casual style fits.
If you are talking to someone respectfully, 심했어요 is safer.

Yes, 고통 can refer to both physical and emotional suffering.

That is one reason it is often translated as pain or suffering depending on context.

For example:

• physical: pain from injury, illness, treatment
• emotional: grief, trauma, mental anguish

If the speaker specifically wants to mean ordinary physical pain, 아픔 or 통증 may also appear depending on context.
Compared with those, 고통 often sounds heavier and more intense.

Both can relate to pain, but they are not exactly the same.

• 고통: suffering, severe pain, anguish; often heavier, more intense
• 아픔: pain, hurt, ache; can sound broader or sometimes softer depending on context

So 고통이 심했어 sounds quite strong, as if the suffering was serious.

Very roughly:

• 아픔이 있었어 = There was pain / hurt
• 고통이 심했어 = The suffering was severe

But actual nuance always depends on context.

Korean often leaves out information that is understood from context.

So 고통이 심했어 literally focuses on the pain itself:

• The pain was severe
• The suffering was intense

The person experiencing it does not need to be stated if it is already clear.

In context, it could mean:

• I was in severe pain
• He was suffering badly
• She went through intense pain

Korean often relies on the surrounding conversation to supply that information.

Yes, but the nuance changes.

• 고통이 심했어 = simple statement of fact
• 고통은 심했어 = the pain, though, was severe / as for the pain, it was severe

Using 은/는 often introduces:

• contrast
• topic marking
• emphasis

For example, if you are comparing different aspects of an experience:

• 치료는 성공적이었지만 고통은 심했어.
• The treatment was successful, but the pain was severe.

So both are possible, but 이 is the more neutral default.

It is commonly pronounced close to 시메써.

A helpful rough guide:

• 고통이 ≈ 고통이
• 심했어 ≈ 시메써

This is because 했어 is pronounced more like 해써 in natural speech.

So the whole sentence sounds roughly like:

• 고통이 시메써

Of course, that is only an approximation in English spelling, but it can help with listening.

Yes. You can add adverbs such as:

• 정말 = really
• 너무 = very / too
• 아주 = very
• 굉장히 = extremely

Examples:

• 고통이 정말 심했어. = The pain was really severe.
• 고통이 너무 심했어. = The pain was so severe.
• 고통이 굉장히 심했어. = The pain was extremely severe.

These all work naturally, though the exact nuance depends on tone and context.