Otkud ti taj stari otvarač?

Otkud literally means from where?, but in this kind of sentence it usually asks about the source of something, not just its location.

So with Otkud ti taj stari otvarač?, the idea is something like:

• Where did you get that old opener?
• How come you have that old opener?

It can also suggest a bit of surprise or curiosity.

Yes. Odakle ti taj stari otvarač? is also natural.

A rough difference is:

• odakle = a bit more neutral / standard
• otkud = very common in speech, sometimes a little more vivid or surprised

In everyday conversation, both are fine.

Here ti is the unstressed dative form meaning to you.

It does not mean the subject you in the same way English uses you. Croatian often uses the dative to talk about possession or something being in someone's possession.

So ti here gives the sense of:

• to you
• with you
• in your possession

That is why the sentence feels like Where did you get... ? or How come you have... ?

Ti and tebi are both dative singular, but they are used differently:

• ti = unstressed clitic form
• tebi = stressed/full form

In a normal neutral sentence, Croatian prefers the clitic:

• Otkud ti taj stari otvarač?

If you want emphasis or contrast, you can use tebi:

• Otkud tebi taj stari otvarač?
  = Where did you get that old opener?

So tebi sounds more emphatic.

This is a very common feature of spoken Croatian. Short questions often leave out a verb when the meaning is obvious from context.

So Otkud ti taj stari otvarač? is a natural elliptical sentence. English usually needs to supply something like:

• Where did you get that old opener?
• How come you have that old opener?

Croatian does not always need to state the verb explicitly in casual speech when the relationship is clear.

The form taj stari otvarač can be either nominative or accusative singular, because otvarač is a masculine inanimate noun, and for that type the nominative and accusative singular look the same.

So the form itself does not show the difference.

That means a learner should focus more on the overall pattern than on trying to identify the case only from the ending here.

Also notice the agreement:

• taj = masculine singular
• stari = masculine singular
• otvarač = masculine singular

They all match.

In Croatian, the normal order is:

• demonstrative + adjective + noun

So:

• taj stari otvarač = that old opener

This is the usual and natural order.
Something like stari taj otvarač would not be normal here.

Because ti is a clitic. Croatian clitics usually go in second position in the clause.

So after the first word Otkud, the clitic ti appears immediately:

• Otkud ti taj stari otvarač?

That placement is very typical of Croatian clitics.

Otvarač means opener.

By itself, it is a fairly general word, so the exact kind depends on context. For example, it could mean:

• a bottle opener
• a can opener
• another kind of opener

If you want to be specific, Croatian often adds more words:

• otvarač za boce = bottle opener
• otvarač za konzerve = can opener

Taj points to a specific opener that the speaker and listener can identify.

It often works like English that:

• taj otvarač = that opener

It may refer to something:

• visible in the situation
• already mentioned
• already known to both people

In this sentence, taj can also add a small sense of Look at that thing / that particular one.

It sounds very natural in everyday spoken Croatian.

It feels somewhat colloquial because:

• otkud is especially common in speech
• the sentence leaves the verb unstated

That does not mean it is wrong. It is completely normal conversation.
A slightly more neutral or standard-sounding version might be:

• Odakle ti taj stari otvarač?

Not necessarily. Stari can mean different things depending on context:

• old in age
• old-fashioned
• worn-out
• familiar / long-owned

So taj stari otvarač could mean an opener that is literally old, but it could also just be an older, worn, or familiar one. Context decides the exact nuance.