gangajiga keuge jijeoseo aiga manhi nollan geos gata.

Because the sentence has two clauses, and each clause has its own subject:

• 강아지가 크게 짖어서 = the puppy barked loudly
• 아이가 많이 놀란 것 같아 = the child seems to have gotten very startled

So it is completely normal to have two -가 markers here. Korean often marks the subject separately in each clause.

Yes, 크다 usually means to be big, but its adverb form 크게 can also mean loudly when talking about sounds.

So:

• 크다 = to be big
• 크게 = bigly/large-ly in form, but naturally loudly in this context

With verbs of sound like 짖다 (to bark), 크게 짖다 means to bark loudly.

Here, -어서 connects the two clauses and shows a reason or cause:

• 강아지가 크게 짖어서 = because the puppy barked loudly / the puppy barked loudly, so...

It often works like:

• because
• so
• and as a result

In this sentence, the barking is the reason the child got startled.

In Korean, non-final verbs often do not show tense as explicitly as English does. The tense is often understood from the whole sentence.

So 짖어서 can still refer to a completed action in context:

• 강아지가 크게 짖어서 아이가 놀랐어 = The puppy barked loudly, so the child got startled.

You usually do not say 짖었어서 here. 짖어서 is the natural form.

놀란 것 같아 uses the pattern:

• verb/adjective in modifier form + 것 같아

Here:

• 놀라다 = to be startled
• 놀란 = startled / having been startled (modifier form)
• 것 같아 = it seems / I think / it looks like

So 아이가 많이 놀란 것 같아 literally means something like:

• It seems that the child was very startled
• I think the child got really startled

If you said 놀랐어, that would be a direct statement: The child got startled.
But 놀란 것 같아 is softer and more uncertain: seems like.

것 같아 means it seems, it looks like, or I think in the sense of making an observation or guess.

It shows that the speaker is not stating the fact with complete certainty. They are inferring it from what they saw or heard.

So:

• 아이가 많이 놀란 것 같아 = It seems the child got really startled

This is very common in Korean when you want to sound less absolute.

같아 is casual/informal speech.

More polite versions would be:

• 강아지가 크게 짖어서 아이가 많이 놀란 것 같아요.
• 강아지가 크게 짖어서 아이가 많이 놀란 것 같습니다. (more formal)

So the original sentence sounds like something you would say to a friend, family member, or someone younger.

많이 comes from 많다 (to be many/a lot), but as an adverb it often means:

• a lot
• very much
• greatly

In this sentence:

• 많이 놀란 = very startled / really shocked

So it does not mean many startled or anything like that. It modifies the verb/adjective-like expression and intensifies it.

Because 놀라다 is an intransitive verb, meaning to be startled.

The child is the one experiencing the feeling, so the child is the subject:

• 아이가 놀라다 = the child gets startled

If you wanted to say someone startled the child, you would use a different structure, such as:

• 아이를 놀라게 하다 = to make the child startled
• 아이를 놀래키다 = to startle the child

So in the original sentence, the child is not the object; the child is the subject of 놀라다.

They are related, but not exactly identical in feeling.

• 개 = dog (general word)
• 강아지 = puppy, little dog, or a cute/pet-like dog

In everyday Korean, 강아지 often sounds softer, cuter, and more affectionate than 개.

So using 강아지 here gives the sentence a more friendly, natural tone.

The sentence is built like this:

• 강아지가 크게 짖어서
  = because the puppy barked loudly

• 아이가 많이 놀란 것 같아
  = it seems the child got very startled

So the overall pattern is:

[cause] + [speaker’s inference about the result]

That makes the sentence feel very natural in conversation: first the cause, then what the speaker thinks happened because of it.