Kad nađem otvarač, otvorit ću konzervu njime.

Because after kad meaning when, Croatian usually uses the present tense to refer to a future event.

So:

• Kad nađem otvarač = When I find the opener
• otvorit ću konzervu = I will open the can

This is very similar to English, where we also say:

• When I find it, I’ll open the can
• not normally When I will find it...

In Croatian, nađem is the present tense of the perfective verb naći, and here it means a future completed action: when I have found / once I find.

Because in Croatian, after time conjunctions like kad meaning when, you normally do not use the future tense to express a future time clause.

So:

• Kad nađem otvarač, otvorit ću konzervu. = correct
• Kad ću naći otvarač... = not right in this meaning

Kad ću naći otvarač? would work only as a separate question meaning:

• When will I find the opener?

So the issue is not that ću naći is wrong in general, but that it is wrong inside this kind of when-clause.

In standard Croatian, the future is formed with the infinitive plus a clitic form of htjeti:

• otvoriti
  • ću

But when the auxiliary ću comes after the infinitive, standard spelling drops the final -i of the infinitive:

• otvorit ću

So these are the standard patterns:

• Otvorit ću konzervu.
• Ja ću otvoriti konzervu.

The form otvoriti ću is very common in informal writing, but standard Croatian prefers otvorit ću.

Because Croatian often leaves out subject pronouns when they are already clear from the verb form.

Here, the sentence means I because of the verb forms:

• nađem = I find
• ću with the future verb = I will

So ja is understood automatically.

You could say:

• Kad nađem otvarač, otvorit ću konzervu njime.

or, with emphasis:

• Kad ja nađem otvarač, otvorit ću konzervu njime.

Adding ja is usually only for contrast or emphasis.

It is in the accusative case here, because it is the direct object of nađem. But for many masculine inanimate nouns, the nominative and accusative singular have the same form.

So:

• nominative: otvarač
• accusative: otvarač

That is why you do not see a change.

Compare that with a masculine animate noun, where the accusative often looks different:

• nominative: pas = dog
• accusative: psa

So in this sentence, otvarač is accusative, even though it looks unchanged.

Because konzervu is the accusative singular of konzerva.

• konzerva = nominative singular
• konzervu = accusative singular

It changes because it is the direct object of otvorit ću:

• otvorit ću što? → konzervu

This is a regular pattern for many feminine nouns ending in -a:

• knjiga → knjigu
• boca → bocu
• konzerva → konzervu

Njime means with it or more naturally using it.

It refers back to otvarač:

• Kad nađem otvarač, otvorit ću konzervu njime.
• When I find the opener, I’ll open the can with it.

So njime is standing in for otvaračem:

• ...otvorit ću konzervu otvaračem.
• ...I’ll open the can with the opener.

This is a very natural way in Croatian to refer back to an already mentioned tool or instrument.

Njime is the instrumental singular form of the pronoun on/ono.

It is used because Croatian often uses the instrumental case to show the means or tool used to do something.

So:

• otvoriti konzervu njime = open the can with it / by means of it

This is the same instrumental idea as:

• olovkom = with a pencil
• nožem = with a knife
• ključem = with a key

Since otvarač is masculine, the pronoun referring to it is njime.

Because njime is the more precise way to express instrument/means.

• njime = with it, using it, by means of it
• s njim = literally with him/it

Croatian often uses the bare instrumental for tools:

• Pišem olovkom. = I write with a pencil.
• Režem nožem. = I cut with a knife.

So otvorit ću konzervu njime sounds like I’ll use it to open the can.

You may also hear s njim or s otvaračem in everyday speech, and people will understand it, but njime is a very neat, grammatical way to express the instrument.

Yes. The verbs here are built from perfective verbs:

• naći → nađem
• otvoriti → otvorit ću

That matters because the sentence describes single completed actions:

1. first I find the opener
2. then I open the can

Perfective verbs are the natural choice for that.

If you used imperfective verbs, the meaning would change and could sound odd here. Imperfective usually suggests:

• an ongoing process
• repeated action
• habitual action
• unfinished action

So this sentence uses perfective forms because it is about one successful finding and one completed opening.

Yes, in this sentence they mean the same thing: when.

• Kad nađem otvarač...
• Kada nađem otvarač...

Both are correct.

The difference is mostly one of style:

• kad is shorter and very common in everyday speech
• kada can sound a bit fuller, sometimes slightly more formal or deliberate

So in this sentence, either one works.

Yes, Croatian word order is fairly flexible, but some versions sound more natural than others.

The original sentence is fine:

• Kad nađem otvarač, otvorit ću konzervu njime.

You could also say:

• Kad nađem otvarač, njime ću otvoriti konzervu.

That version puts more focus on njime.

Croatian allows this flexibility because cases show grammatical roles, but clitics like ću still follow special placement rules, usually near the beginning of their clause. So not every rearrangement sounds equally natural.

For a learner, the safest natural versions are:

• Kad nađem otvarač, otvorit ću konzervu njime.
• Kad nađem otvarač, njime ću otvoriti konzervu.

By itself, otvarač means opener.

Depending on context, it could mean:

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

But in this sentence, because of konzervu (a can), it is naturally understood as a can opener.

So the object konzerva tells you what kind of opener is meant.

Two sounds here often catch English speakers:

• đ in nađem
• nj in njime

A rough guide:

• đ is a soft sound, somewhat like the j sound in judge, but lighter
• nj is like the ny sound in canyon or Spanish ñ

So very roughly:

• nađem ≈ NAH-jem with a soft dj/j sound
• njime ≈ NYEE-meh

These are only approximations, but they can help you get close.