Mám pro sestru ještě jeden dárek, ale zatím nevím, kdy jí ho dám.

Because pro always takes the accusative case, so sestra becomes sestru.

• pro sestru = for (my) sister
• sestře is dative, and that would be used in a different structure, for example:
  • Dám sestře dárek. = I’ll give my sister a gift.

So in this sentence:

• Mám pro sestru ještě jeden dárek literally means I have one more gift for my sister.

The idea is not I have my sister something, but I have something intended for my sister.

In Czech, possessives are often omitted when they are obvious from context.

So pro sestru can naturally mean for my sister if the speaker is clearly talking about their own sister. Czech does this much more often than English.

You could say pro moji sestru, but it would sound more explicit or contrastive, for example if you needed to stress whose sister you mean.

Ještě jeden means one more, another one, or an additional one.

• jeden = one
• ještě here adds the sense of still / yet / one more

So:

• ještě jeden dárek = one more gift / another gift

This is very common in Czech:

• Ještě jednu otázku. = One more question.
• Dám si ještě jedno pivo. = I’ll have one more beer.

Because dárek is the direct object of mám, and in this sentence it is in the accusative singular. For this masculine inanimate noun, the accusative singular has the same form as the nominative singular.

• nominative: dárek
• accusative: dárek

So although the case changes grammatically, the form stays the same.

Here zatím means for now or so far.

So:

• ale zatím nevím = but for now I don’t know yet

It suggests that the speaker may know later, but does not know at this moment.

Be careful not to confuse it with ještě:

• ještě nevím = I don’t know yet
• zatím nevím = for now / so far I don’t know

In many contexts, both are possible, but zatím adds more of a current-state-for-now feeling.

Because kdy means when, and after nevím you use kdy to introduce an indirect question:

• Nevím, kdy jí ho dám. = I don’t know when I’ll give it to her.

Až is different. It usually means when / once / after in the sense of a future event that is expected to happen:

• Dám jí ho, až ji uvidím. = I’ll give it to her when I see her.

So:

• nevím, kdy... = I don’t know when...
• až... = when/once... in a future time clause

They are not interchangeable here.

Jí is the dative singular form of ona (she / her), and here it means to her.

In the clause:

• kdy jí ho dám

the verb dát often takes:

• a direct object in the accusative = ho = it
• an indirect object in the dative = jí = to her

So the structure is:

• dám ho jí / dám jí ho = I’ll give it to her

In natural Czech, the clitic order strongly favors jí ho.

Ho is a pronoun meaning him/it in the accusative singular, used here for a masculine inanimate noun, namely dárek.

Since dárek is masculine inanimate, the matching pronoun is:

• ho = it

You would use to for a neuter noun, not for dárek.

Examples:

• Mám dárek. Dám ho sestře. = I have a gift. I’ll give it to my sister.
• Mám auto. Prodám ho. = I have a car. I’ll sell it.

Because Czech has preferred word order rules for short unstressed pronouns, often called clitics.

In a sequence of object pronouns, the usual order is:

1. dative pronoun
2. accusative pronoun

So:

• jí ho dám = natural
• ho jí dám = much less natural in normal neutral speech

Also, these short pronouns tend to appear near the beginning of the clause, typically in the second-position area.

That is why kdy jí ho dám sounds normal.

This is because dát is a perfective verb. In Czech, the present tense forms of perfective verbs usually refer to the future.

So:

• dám literally looks like present tense in form
• but in meaning it is I will give

This is normal with perfective verbs:

• napíšu = I will write
• udělám = I will do
• přijdu = I will come

By contrast, budu is used to form the future of imperfective verbs.

The difference is mainly aspect.

• dám is perfective: it focuses on the gift being given as a completed event.
• budu dávat is imperfective future: it suggests an ongoing action, repetition, process, or a less bounded event.

In this sentence, the speaker means a single completed act of giving, so dám is the natural choice.

Compare:

• Nevím, kdy jí ho dám. = I don’t know when I’ll give it to her.
• Budu jí dávat dárky každý rok. = I’ll be giving her gifts every year.

Because Czech usually omits subject pronouns when the verb form already shows who the subject is.

Here:

• mám = I have
• nevím = I don’t know
• dám = I will give

So já is unnecessary unless you want to emphasize it.

For example:

• Já nevím can mean I don’t know with extra emphasis, maybe contrasting with someone else.
• Neutral Czech usually just says Nevím.

Yes, that is also correct.

• Mám pro sestru ještě jeden dárek
• Mám ještě jeden dárek pro sestru

Both mean the same basic thing, but the emphasis shifts slightly.

• pro sestru earlier in the sentence can highlight who the gift is for
• ještě jeden dárek earlier can highlight the fact that there is another gift

Czech word order is flexible, and speakers often move parts of the sentence to reflect emphasis, topic, or rhythm.

Ale means but, and it connects two ideas in contrast:

1. I have one more gift for my sister
2. I don’t know when I’ll give it to her

The contrast is:

• the speaker has the gift already,
• but doesn’t know yet when the giving will happen.

So ale is a natural coordinating conjunction here.

Not in this sentence.

Here you need the unstressed dative pronoun jí because it functions as to her with the verb dám.

• jí = to her / her
• ní is used after prepositions or in stressed positions in other constructions

For example:

• Dám jí dárek. = I’ll give her a gift.
• Mluvím o ní. = I’m talking about her.

So in kdy jí ho dám, jí is the correct form.