Jedan novčić je pao ispod pulta, pa ga nisam mogla odmah uzeti.

Both are possible, depending on context.

Croatian has no articles like a/an/the, so jedan literally means one, but it is also often used where English would simply say a when introducing something new.

So:

• Jedan novčić can mean one coin
• but in many contexts it is naturally translated as a coin

If you wanted a less specific version, you could also say Novčić je pao..., but jedan makes the noun feel a bit more introduced or singled out.

This is the normal Croatian past tense structure:

• auxiliary biti in the present tense
• plus the l-participle of the main verb

So here:

• je = is / auxiliary for he/it
• pao = past participle of pasti (to fall)

Together, je pao means fell / has fallen depending on context.

So:

• pasti = to fall
• je pao = fell

Because pao agrees with the subject novčić.

In Croatian, the past participle agrees in:

• gender
• number

Novčić is masculine singular, so the form is pao.

Compare:

• Novčić je pao = the coin fell
• Kovanica je pala = the coin fell (kovanica is feminine)
• Dijete je palo = the child fell (dijete is neuter)

Because ispod requires the genitive case.

So:

• base form: pult
• after ispod: pulta

This is a very common Croatian pattern:

• ispod stola = under the table
• ispod kreveta = under the bed
• ispod pulta = under the counter

So the learner takeaway is:

• ispod
  • genitive

Here pa means something like:

• so
• and so
• then
• so then

It connects the two ideas:

• the coin fell under the counter
• as a result, I couldn’t take it right away

So in this sentence, pa has a result/continuation feeling, not just a simple and.

Here ga means it, referring back to novčić.

Even though novčić is grammatically masculine, that does not mean the English translation must be him. For objects, English usually uses it.

Grammatically:

• ga is the short unstressed form of the accusative pronoun
• it is used because the coin is the direct object of uzeti (to take)

So:

• uzeti novčić = to take the coin
• uzeti ga = to take it

This means I could not / I wasn’t able to.

It is built from:

• nisam = I am not / I did not as the negative auxiliary form for I
• mogla = past participle of moći (to be able to / can), feminine singular

So:

• mogla sam = I was able / I could
• nisam mogla = I was not able / I couldn’t

A native English speaker should especially notice that Croatian does not say ne sam. The negative form is a single word:

• sam → nisam
• si → nisi
• je → nije

Mogla is feminine singular, so it tells you the speaker is female.

That is because in Croatian, in the past tense, the participle agrees with the subject, even when the subject is just implied.

Here the implied subject is I, so:

• if a woman says it: nisam mogla
• if a man says it: nisam mogao

This agreement is very important in Croatian past-tense sentences.

Because moći works like a modal verb, and modal verbs in Croatian are commonly followed by an infinitive.

So:

• mogu uzeti = I can take
• mogla sam uzeti = I could / was able to take
• nisam mogla uzeti = I couldn’t take

This is similar to English:

• can take
• could take

Because uzeti is perfective, and here the action is seen as one complete act: taking the coin.

Croatian often chooses:

• perfective for a single completed action
• imperfective for ongoing, repeated, or habitual action

So here:

• uzeti = to take, pick up, get hold of once
• uzimati = to be taking, to take repeatedly, to keep taking

Since the idea is I couldn’t pick it up right away, uzeti is the natural choice.

Because odmah is an adverb, and Croatian word order is fairly flexible.

Here it most naturally modifies the action uzeti:

• nisam mogla odmah uzeti = I couldn’t take it immediately

Croatian often places adverbs near the verb or infinitive they belong to. Other placements can also be possible, but this one sounds very natural.

Because je and ga are clitics: short, unstressed forms that usually appear near the beginning of the clause.

So Croatian does not always follow English-style word order. These little words tend to move into early positions:

• Jedan novčić je pao...
• pa ga nisam mogla...

This is one of the biggest word-order differences between English and Croatian. A good beginner rule is:

• short unstressed forms such as je, ga, sam, si, smo usually come early, not wherever English would place them

Because pa here joins two full clauses:

1. Jedan novčić je pao ispod pulta
2. ga nisam mogla odmah uzeti

When pa means something like so / and so / then and introduces another clause with its own verb, a comma is very common and standard.

So the comma helps show the sentence has two linked parts, not just one simple phrase.