ginjanghaessjiman gwaenchanheun cheokhaesseo.

-지만 means but / although.

So 긴장했지만 connects the first idea to the second:

• 긴장했지만 = I was nervous, but...

It attaches to the verb or adjective stem:

• 긴장하다 → 긴장했지만
• 예쁘다 → 예쁘지만

In this sentence, it shows contrast: the speaker was nervous, but acted like they were okay.

It comes from the verb 긴장하다, which means to be nervous or to get tense.

Here is the breakdown:

• 긴장하다 = to be nervous
• 긴장했- = past tense form (was nervous / got nervous)
• 긴장했지만 = was nervous, but...

So:

• 긴장하다 → 긴장했어 = was nervous
• 긴장했어
  • -지만 → 긴장했지만 = was nervous, but

The 했 part comes from the past tense of 하다.

Because 척하다 usually takes a modifier form before it.

Here, 괜찮다 is being used to describe the kind of pretense:

• 괜찮은 척 = the act of pretending to be okay

With adjectives in Korean, the modifying form is often:

• 괜찮다 → 괜찮은

So this pattern is:

• adjective/verb modifier + 척하다

Examples:

• 모르는 척하다 = to pretend not to know
• 아는 척하다 = to pretend to know
• 괜찮은 척하다 = to pretend to be okay

So 괜찮아 척 is not the correct structure here.

척하다 means to pretend, to act as if, or to make it look like.

It is very common in Korean.

Examples:

• 아는 척하다 = to pretend to know
• 못 본 척하다 = to pretend not to see
• 바쁜 척하다 = to pretend to be busy
• 괜찮은 척하다 = to pretend to be okay

In your sentence, 괜찮은 척했어 means the speaker acted like they were okay, even though they were actually nervous.

했어 is the informal casual past tense form of 하다.

Since 척하다 is the main verb here, it gets conjugated:

• 척하다 → 척했어

So:

• 괜찮은 척했어 = pretended to be okay
• more literally: did the act of seeming okay

The ending -어 shows casual speech, so this sentence sounds like something you would say to a friend, someone younger, or in a diary/self-talk style.

Yes, that is a very natural way to understand it.

More literally, it is something like:

• I did an okay-seeming act
• I acted as if I was okay

But in natural English, you would usually say:

• I pretended I was okay
• I acted like I was okay
• I put on a brave face (depending on context)

So 척하다 often sounds a little like acting rather than just internally pretending.

This is a very common point of confusion.

In Korean, the form before a noun-like element such as 척 is often a modifier, and it does not have to match English tense in a simple one-to-one way.

• 괜찮은 척했어 = pretended to be okay
• literally, did the pretending of being okay

The pretending happened in the past, which is shown by 했어.

The 괜찮은 part describes the state the speaker was pretending to have: being okay.

If you changed the modifier, you would change the nuance:

• 괜찮은 척했어 = pretended to be okay
• 괜찮았던 척했어 = pretended that I had been okay / pretended I used to be okay

So the sentence uses 괜찮은 because the pretense is about the state okay.

In Korean grammar, 괜찮다 is usually treated as a descriptive verb (often called an adjective in learner-friendly explanations).

That is why it changes like this when it modifies something:

• 괜찮다 → 괜찮은

This is similar to:

• 예쁘다 → 예쁜
• 좋다 → 좋은

So even though it behaves like an adjective in meaning, in Korean grammar it is part of the predicate system and conjugates.

It suggests that the speaker was hiding their true feelings.

So the nuance is not just:

• I was nervous

but rather:

• I was nervous, but I tried not to show it
• I kept up appearances
• I made myself look fine

괜찮은 척했어 often implies emotional control, social pressure, embarrassment, or not wanting others to worry.

Depending on context, it can sound:

• vulnerable
• reflective
• slightly sad
• mature or restrained

Yes. Korean often omits the subject when it is clear from context.

So 긴장했지만 괜찮은 척했어 could mean:

• I was nervous, but I acted like I was okay
• He/She was nervous, but acted like they were okay

In many real conversations, the listener knows who the sentence is about from the situation.

If you wanted to make it explicit, you could say:

• 나는 긴장했지만 괜찮은 척했어.
• 걔는 긴장했지만 괜찮은 척했어.

It is casual, non-polite speech because it ends in -어:

• 척했어

This is appropriate with:

• close friends
• younger people
• family
• diary-style writing
• internal narration

A polite version would be:

• 긴장했지만 괜찮은 척했어요.

A more formal/written style could be:

• 긴장했지만 괜찮은 척했다.

So the sentence you have sounds natural and conversational.

Yes, you often can, but the nuance is a little different.

• 긴장했지만 focuses more on I was nervous / I felt nervous
• 긴장됐지만 often feels more like I became nervous / I felt nervous in a slightly more spontaneous or state-focused way

In many everyday situations, both are possible.

For example:

• 긴장했지만 괜찮은 척했어.
• 긴장됐지만 괜찮은 척했어.

Both are natural, but 긴장했지만 may feel a bit more direct and active in some contexts.

Not always, but it often carries the idea that the appearance is not genuine.

So it can sound:

• neutral: simply pretending
• slightly negative: being fake, showing off, hiding the truth
• sympathetic: trying to stay strong

Examples:

• 아는 척하지 마. = Don’t pretend you know.
• 괜찮은 척했어. = I pretended to be okay.

In your sentence, it does not sound harsh. It sounds more like the speaker is admitting they were trying to hide their nervousness.