onraineuro jumunharyeogo haessneunde nae saijeuneun imi pumjeorieosseo.

온라인으로 means online or more literally by/through online means.

The particle -으로 often marks:

• a method or means
• a direction
• a tool

In this sentence, it shows the method of ordering:

• 온라인으로 주문하다 = to order online
• literally: to place an order through online

A learner might expect 온라인에서, but that would sound more like at/on the online place/platform. 온라인으로 is very natural when talking about the way something is done.

This part breaks down like this:

• 주문하다 = to order
• 주문하려고 하다 = to intend to order / to be about to order / to try to order
• 주문하려고 했는데 = I was going to order, but...

So -(으)려고 하다 expresses intention or plan.

Examples:

• 먹으려고 해 = I’m going to eat / I intend to eat
• 가려고 했어 = I was going to go

In this sentence, 주문하려고 했는데 means:

• I was trying / planning to order it, but...

The -는데 at the end sets up a contrasting result.

-는데 connects the first clause to what comes next. It often gives a feeling like:

• but
• and then
• however
• so, the situation was...

Here it strongly suggests contrast:

• 온라인으로 주문하려고 했는데 = I was going to order online, but
• 내 사이즈는 이미 품절이었어 = my size was already sold out

So 했는데 is important because it links the expectation and the disappointing result.

내 사이즈는 literally means as for my size.

In English, we usually say:

• my size
• my size was sold out

In Korean, the thing being talked about is often omitted if it is obvious from context. So 내 사이즈는 could mean:

• my size in that item
• my clothing size
• my shoe size

The omitted noun is understood from the situation.

So this sentence does not need to say:

• 내 신발 사이즈는
• 내 옷 사이즈는

unless the speaker wants to be extra specific.

Great question. Both are possible in some situations, but they feel different.

• 내 사이즈는 = as for my size
• 내 사이즈가 = my size as the subject/new information

Using -는 gives a topic or contrast feeling. It can imply something like:

• As for my size, it was already sold out
• maybe other sizes existed, but mine was gone

This makes sense in a shopping context because there is often an implied contrast:

• other colors were available
• other sizes were available
• my size, however, was sold out

So -는 sounds very natural here.

이미 means already.

So:

• 품절이었어 = was sold out
• 이미 품절이었어 = was already sold out

It emphasizes that by the time the speaker tried to order, it was too late.

Compare:

• 내 사이즈는 품절이었어 = my size was sold out
• 내 사이즈는 이미 품절이었어 = my size was already sold out

The second sounds a little more disappointed or final.

품절 means sold out.

It is a noun, but in Korean it is often used with 이다:

• 품절이다 = to be sold out

So:

• 품절이야 = it’s sold out
• 품절이었어 = it was sold out
• 품절됐어 = it got sold out / became sold out

A useful distinction:

• 품절이다 focuses on the state
• 품절되다 focuses more on the change into that state

In this sentence, 품절이었어 simply describes the condition of the size when the speaker checked.

Because 품절 is a noun, and it combines with 이다:

• 품절이다 → past tense → 품절이었다 / 품절이었어

Since 품절 ends in a consonant, you keep the 이:

• 학생이다 → 학생이었어
• 품절이다 → 품절이었어

You get the shortened 였어 form after a noun ending in a vowel:

• 의사이다 → 의사였어
• 회사이다 → 회사였어

So 품절이었어 is correct.

Yes. The final ending -었어 is casual/informal.

• 품절이었어 = casual
• 품절이었어요 = polite
• 품절이었습니다 = formal

So the full sentence is something you might say:

• to a friend
• in casual conversation
• in a text message

A polite version would be:

• 온라인으로 주문하려고 했는데 제 사이즈는 이미 품절이었어요.

내 is the casual form of my. 제 is the humble/polite form.

So:

• 내 사이즈 = my size, casual
• 제 사이즈 = my size, polite

Since the sentence ends casually with -었어, using 내 matches the tone.

If you were speaking politely, you would normally say:

• 제 사이즈는 이미 품절이었어요

It can suggest either, depending on context.

Most literally:

• I was going to order
• I intended to order

But in natural English, because the next clause shows a problem, it may be translated more naturally as:

• I tried to order online, but...

So the Korean itself is built from intention, but in context it can feel close to trying.

A very literal breakdown would be:

• 온라인으로 = online / through online means
• 주문하려고 했는데 = I was going to order, but
• 내 사이즈는 = as for my size
• 이미 = already
• 품절이었어 = was sold out

So a literal version is:

I was going to order online, but as for my size, it was already sold out.

A more natural English version would be:

I tried to order online, but my size was already sold out.

Some parts can move, but the original order is very natural.

Original:

• 온라인으로 주문하려고 했는데 내 사이즈는 이미 품절이었어.

Possible variation:

• 내 사이즈는 이미 품절이어서 온라인으로 주문하려고 했지만 못 했어.

But that becomes more explicit and changes the nuance.

In the original sentence, the flow is:

1. what the speaker planned to do
2. the disappointing result

That order feels very natural in everyday Korean.