helmesi eobseoseo oneureun jajeongeoreul an tasseo.

A very common Korean pattern is:

[reason] + [time/topic] + [object] + [verb]

So here:

• 헬멧이 없어서 = because I didn’t have a helmet
• 오늘은 = as for today / today
• 자전거를 = a bicycle
• 안 탔어 = didn’t ride

Korean often puts the reason first, then the main action afterward.

With 있다 and 없다, the thing that exists or does not exist usually takes 이/가.

So:

• 헬멧이 있다 = there is a helmet / I have a helmet
• 헬멧이 없다 = there is no helmet / I don’t have a helmet

That is why it is 헬멧이 없어서, not 헬멧을 없어서.

Yes, literally 없다 means to not exist / to not be there. But Korean also uses 있다/없다 to express possession.

For example:

• 돈이 있어요 = I have money
• 돈이 없어요 = I don’t have money

So 헬멧이 없어서 can naturally mean because I didn’t have a helmet.

-아서/어서 connects two clauses and often means because, so, or and then, depending on context.

Here it shows a reason:

• 헬멧이 없어서 = because there was no helmet / because I didn’t have a helmet

So the whole sentence means that the lack of a helmet is the reason for not riding the bicycle.

This is a very common question.

In Korean, the first clause often does not need past marking if the time is already clear from the main clause. So:

• 헬멧이 없어서 오늘은 자전거를 안 탔어
  naturally means
  Because I didn’t have a helmet, I didn’t ride a bike today.

Even though 없어서 is not marked as past, the past meaning is understood from 안 탔어.

A form like 없었어서 is usually awkward and not the normal choice here.

The 는 is the topic marker. It adds a slight sense of contrast or emphasis.

So:

• 오늘 자전거를 안 탔어 = I didn’t ride a bike today.
• 오늘은 자전거를 안 탔어 = As for today, I didn’t ride a bike.

Using 오늘은 can suggest something like:

• maybe on other days I did ride
• but today was different

That contrast can be very subtle.

In Korean, 타다 takes the thing you ride as its direct object.

So you say:

• 버스를 타다 = ride/take the bus
• 택시를 타다 = take a taxi
• 자전거를 타다 = ride a bicycle

That is why the sentence has 자전거를.

It comes from 타다:

1. verb stem: 타-
2. past ending: -았어
3. 타 + 았어
4. this contracts to 탔어

So:

• 타다 = to ride
• 탔어 = rode

And with 안:

• 안 탔어 = didn’t ride

안 is a very common way to make a verb negative in Korean. It goes directly before the verb.

• 타다 = to ride
• 안 타다 = to not ride
• 안 탔어 = didn’t ride

So 안 here is simply the negation word, similar to not in English.

These are close, but the nuance is different:

• 안 탔어 = didn’t ride
• 못 탔어 = couldn’t ride

In this sentence, 안 탔어 sounds like the speaker chose not to ride because there was no helmet.

If you wanted to emphasize inability or circumstances preventing it, 못 탔어 would also be possible:

• 헬멧이 없어서 오늘은 자전거를 못 탔어
  = I couldn’t ride my bike today because I didn’t have a helmet.

So 안 탔어 is more like a simple negative fact, while 못 탔어 stresses that it wasn’t possible.

Korean very often leaves out the subject when it is obvious from context.

So even though there is no 나 or 저, the meaning can still be:

I didn’t ride a bike today because I didn’t have a helmet.

If you added the subject, it would be something like:

• 나는 헬멧이 없어서 오늘은 자전거를 안 탔어.

But in normal conversation, leaving it out is very natural.

탔어 is in the casual informal style. It is natural with:

• friends
• younger people
• people you are close to

A more polite version would be:

• 헬멧이 없어서 오늘은 자전거를 안 탔어요.

So the original sentence sounds like everyday spoken Korean between people who are on casual terms.