Poljubit ću baku kad dođe na ručak.

Because ću is an unstressed clitic, and clitics normally cannot stand at the very beginning of a sentence.

So you can say:

• Ja ću poljubiti baku.
• Poljubit ću baku.

But not normally:

• Ću poljubiti baku.

In Poljubit ću, the infinitive poljubiti comes before the clitic ću, and in that position the final -i of the infinitive is dropped: poljubit ću.

This is a normal spelling and grammar pattern in Croatian future tense.

When the infinitive stands before the future clitic (ću, ćeš, će, ćemo, ćete, će), infinitives ending in -ti usually drop the final -i:

• pisati → pisat ću
• kupiti → kupit ću
• poljubiti → poljubit ću

So Poljubit ću is the expected form here.

Because baku is the accusative singular of baka.

Here, baka is the direct object of poljubiti (to kiss), so Croatian uses the accusative case:

• nominative: baka = grandma
• accusative: baku = grandma, as the object

Compare:

• Baka dolazi. = Grandma is coming.
• Poljubit ću baku. = I will kiss grandma.

In Croatian, after kad/kada when talking about a future event, it is very common to use the present tense of a perfective verb, not a future form.

So Croatian says:

• kad dođe = when she comes

rather than:

• kad će doći

In standard Croatian, kad će doći is usually not the natural choice in a sentence like this. English uses future less strictly here too: when she comes, not when she will come.

Yes, dođe is formally the 3rd person singular present of the perfective verb doći (to come / arrive).

But because it appears after kad and refers to a future event, it has a future-time meaning:

• kad dođe = when she comes / when she arrives

This is a very common Croatian pattern:

• Nazvat ću te kad stignem.
• Vidjet ćemo se kad završi posao.

So the form is present, but the time reference is future.

Because the sentence refers to one completed arrival.

• doći / dođe is perfective: it focuses on arriving as a completed event.
• dolaziti / dolazi is imperfective: it often suggests repeated action, process, or a more general/habitual meaning.

Here the idea is:

• first she arrives,
• then I kiss grandma.

So perfective dođe fits naturally.

If you used dolazi, it would sound different in meaning or less natural in this context.

The preposition na here takes the accusative and often expresses going/coming to an event, activity, or meal.

So:

• na ručak = for lunch / to lunch

This is similar to expressions like:

• ići na posao = to go to work
• doći na sastanak = to come to a meeting
• pozvati nekoga na večeru = to invite someone to dinner

So dođe na ručak means she comes for the lunch occasion.

Yes. Kad and kada both mean when.

• kad is shorter and very common in everyday speech
• kada is slightly fuller or more formal in tone

So both are correct:

• Poljubit ću baku kad dođe na ručak.
• Poljubit ću baku kada dođe na ručak.

Yes, Croatian word order is fairly flexible, though not completely free.

You could also say:

• Kad dođe na ručak, poljubit ću baku.
• Baku ću poljubiti kad dođe na ručak.
• Ja ću poljubiti baku kad dođe na ručak.

The basic meaning stays the same, but the emphasis shifts:

• Poljubit ću baku... puts focus early on the action
• Baku ću poljubiti... emphasizes grandma
• Kad dođe na ručak... foregrounds the time condition

Croatian does not have articles like English the and a.

So baku can mean things like:

• grandma
• the grandma
• my grandma

depending on context.

In real life, baku here would usually be understood as grandma / my grandma, because that is the most natural interpretation. Croatian often leaves that kind of information to context instead of marking it with an article.

Yes. This is mainly an aspect difference.

• poljubiti is perfective: to kiss once / to give a kiss
• ljubiti is imperfective: to kiss, often in a more ongoing, repeated, or general sense

In this sentence, a single future action is meant, so poljubiti is the natural choice:

• Poljubit ću baku... = I’ll kiss grandma

If you used ljubiti, it would suggest a different kind of action or viewpoint.