Poslije cijepljenja dobio je malu poslasticu i odmah počeo veselo mahati repom.

Because poslije means after and normally takes the genitive case.

• base form: cijepljenje = vaccination
• genitive singular: cijepljenja

So poslije cijepljenja literally means after the vaccination or more naturally after being vaccinated, depending on context.

No. A very common alternative is nakon cijepljenja.

Both are natural:

• poslije cijepljenja
• nakon cijepljenja

They mean essentially the same thing here. Poslije can sound a bit more everyday and conversational, while nakon can sound slightly more neutral or formal, but both are very common.

In Croatian past tense, the auxiliary je is a clitic, and clitics usually go in the second position of the clause.

So in dobio je malu poslasticu, the first stressed word is dobio, and je comes right after it.

You can also hear:

• On je dobio malu poslasticu.

Here je is still in second position, after On.

English speakers often expect the auxiliary first, but Croatian word order works differently. Dobio je is completely normal.

Dobio is the masculine singular past participle of dobiti = to get / receive.

Croatian past tense is usually formed with:

• the auxiliary biti in the present tense
• plus the past participle

So:

• dobio je = he got / he received

The form dobio shows that the subject is masculine singular. That is one reason Croatian often does not need to say on = he.

Because Croatian often drops subject pronouns when the subject is already clear from the verb form or the context.

Here, dobio and počeo are both masculine singular forms, so we understand the subject as he.

Croatian prefers leaving out pronouns unless there is a reason to emphasize them:

• Dobio je malu poslasticu. = He got a small treat.
• On je dobio malu poslasticu. = He got a small treat.
  This version can sound more emphatic, as if contrasting him with someone else.

Because it is the direct object of dobio je = he got, so it goes in the accusative case.

Dictionary forms:

• mala = small
• poslastica = treat, delicacy

Accusative singular feminine:

• malu
• poslasticu

So:

• mala poslastica = a small treat
• dobio je malu poslasticu = he got a small treat

The adjective and noun both change to match gender, number, and case.

Not exactly, though they can overlap.

• slatkiš usually means a sweet / candy
• poslastica is more like a treat, something special and enjoyable

In a sentence about a dog, mala poslastica sounds very natural for a small treat. It does not have to mean candy in the human sense. It can mean a little reward.

Because after početi = to begin / start, Croatian commonly uses the infinitive of the next verb.

So:

• počeo mahati = started to wag / started waving

This is similar to English started to wag.

You do not use two past-tense finite verbs here. So not something like počeo je mahao. The normal pattern is:

• počeo je + infinitive

Because veselo is an adverb, and adverbs modify verbs.

• veseo = cheerful, happy
  This is an adjective.
• veselo = cheerfully, happily
  This is an adverb.

Since it describes how he started wagging his tail, Croatian uses the adverb:

• veselo mahati = to wag happily / cheerfully

Because the verb mahati normally takes the thing being waved in the instrumental case.

• rep = tail
• repom = with the tail / tail-instrumental

So mahati repom literally means to wave with the tail.

In natural English, we would usually say wag the tail or simply wag his tail, but Croatian uses a different structure. This is a very useful expression to learn as a whole:

• mahati repom = to wag one’s tail

Not exactly. It matches the meaning, but the structure is different.

Croatian expression:

• mahati repom = literally to wave with the tail

Natural English equivalent:

• to wag his tail

So if you translate word-for-word, it sounds unusual in English, but in Croatian it is perfectly normal and idiomatic.

Yes, aspect is part of why this sentence sounds natural.

• dobio comes from dobiti, a perfective verb: a completed event
  He got the treat.
• počeo comes from početi, also perfective: the beginning of an action
  He started.
• mahati is imperfective: an ongoing or repeated action
  He started wagging.

This combination makes good sense:

1. one completed event happened
2. then another completed event began
3. then the ongoing action is described

So the sentence presents a natural sequence:

• after the vaccination
• he got a small treat
• and immediately began wagging his tail happily

Odmah means immediately / right away.

It tells you that the second action followed very quickly:

• i odmah počeo veselo mahati repom = and immediately started wagging his tail happily

Its position before počeo is very natural. Croatian word order is flexible, but this placement clearly emphasizes the immediacy of the reaction.