Pas je mirno hodao uz mene na povodcu, ali se mačka kod kuće opet počela grebati.

Croatian normally forms the past tense with:

• a present-tense form of biti (sam, si, je, smo, ste, su)
• plus the l-participle of the main verb

So:

• pas je hodao = the dog walked / was walking

Here, hodao is the masculine singular form, because pas is masculine.

This is the standard everyday past tense in Croatian. English uses one word like walked, but Croatian usually uses two parts.

Because mirno is an adverb, and it describes how the dog was walking.

• miran pas = a calm dog
  
  • here miran is an adjective describing the noun pas
• pas je mirno hodao = the dog walked calmly
  
  • here mirno describes the verb hodao

A very common pattern is:

• adjective: miran
• adverb: mirno

So mirno hodao means walked calmly.

After the preposition uz, Croatian uses the accusative.

So:

• ja = I
• mene = me (accusative/genitive form, and also the form used after prepositions)

That is why you get uz mene, not uz ja.

Also, after a preposition, Croatian uses the full/stressed pronoun form, so you say:

• uz mene
• not uz me

In this sentence, uz mene means something like alongside me / beside me.

These are similar, but not identical:

• uz mene = right by me, alongside me, often with a sense of close physical position
• pored mene = beside me / next to me
• sa mnom = with me

So in this sentence, uz mene works well because it describes the dog's position while walking.

Compare:

• Pas je hodao uz mene. = The dog was walking right alongside me.
• Pas je bio sa mnom. = The dog was with me.
• Pas je stajao pored mene. = The dog was standing next to me.

Na povodcu is the normal Croatian way to say on a leash.

Here, povodcu is the locative singular of povodac.

So:

• nominative: povodac
• locative: povodcu

With na, Croatian can use different cases:

• na + accusative = movement onto something
• na + locative = location/state on something

In na povodcu, the idea is a state: the dog was on a leash, not moving onto a leash. So the locative is used.

This phrase is idiomatic, so it is best learned as a chunk:

• pas je na povodcu = the dog is on a leash

Because se is a clitic, and Croatian clitics usually appear very early in the clause, often in the so-called second position.

So although se belongs in meaning with grebati (to scratch oneself), it often moves forward:

• mačka se počela grebati
• after ali: ali se mačka počela grebati

That is why the sentence has:

• ali se mačka kod kuće opet počela grebati

and not necessarily a version with se directly attached to grebati.

This is very normal Croatian word order.

This is a very good question.

Normally, yes, you would expect:

• mačka je počela = the cat started

But with reflexive se, modern Croatian normally omits the 3rd person singular auxiliary je.

So instead of:

• mačka se je počela grebati

Croatian normally says:

• mačka se počela grebati

This is the standard natural form.

So in your sentence:

• ali se mačka kod kuće opet počela grebati

the missing je is not a mistake. It is exactly what you expect with se in this structure.

A useful comparison:

• Mačka je počela jesti. = The cat started eating.
• Mačka se počela grebati. = The cat started scratching itself.

Because only one reflexive marker is needed.

The verb here is effectively grebati se = to scratch oneself. But when there is a finite verb plus an infinitive, the se often moves upward in the sentence:

• mačka se počela grebati

It is still understood with grebati, even though it appears earlier.

So Croatian does not say:

• mačka se počela grebati se

That would be wrong.

In standard Croatian, after verbs like početi (to begin/start), the normal construction is verb + infinitive:

• počela grebati
• počeo raditi
• počeli govoriti

So počela grebati is the standard Croatian pattern.

A da + present tense structure is much more associated with Serbian and is generally not the preferred standard Croatian choice here.

So for Croatian, learn:

• početi + infinitive

Because kod kuće means at home, while u kući means more literally in the house.

So:

• kod kuće = at home
• u kući = inside the house/building

In many situations, English uses at home, and Croatian naturally uses kod kuće.

Examples:

• Ostajem kod kuće. = I’m staying at home.
• On je kod kuće. = He is at home.
• On je u kući. = He is in the house.

In your sentence, kod kuće sounds natural because it refers to the cat being at home.

Yes, opet means again, but it often also carries the feeling of once again or yet again, depending on context.

So here it suggests that the cat had done this before and has started doing it another time.

Compare:

• Opet pada kiša. = It’s raining again.
• Opet kasniš. = You’re late again.
• Mačka se opet počela grebati. = The cat started scratching itself again.

So opet often adds a sense of repetition, and sometimes mild annoyance.

Because the past participle agrees with the subject in gender and number.

• pas is masculine singular, so: hodao
• mačka is feminine singular, so: počela

Compare:

• pas je hodao
• mačka je hodala
• mačka se počela grebati
• pas se počeo češati

This agreement is a very important part of Croatian past tense.

Not really in the best way.

• hodati = to walk
• ići = to go

So pas je hodao specifically tells you that the dog was walking. If you used išao, the meaning would be more general: went / was going.

In this sentence, hodao is the right choice because the sentence describes the dog’s manner of movement.