Ako ostaneš nestrpljiv, svaka minuta čekanja činit će ti se dužom nego što stvarno jest.

Because Croatian normally uses the present tense after ako for a future condition.

So:

• Ako ostaneš nestrpljiv... = If you remain / become impatient...
• not something like Ako ćeš ostati...

Also, ostati is a perfective verb. Its present-tense forms often refer to a future completed event/state in subordinate clauses like this. So ostaneš here naturally points to what will happen in the future if that condition is met.

It can lean toward remain/stay impatient, but in context it may feel close to end up being impatient.

The verb ostati basically means to remain, stay. With an adjective, it often describes continuing in a state:

• ostati miran = to remain calm
• ostati sam = to remain alone

So ostaneš nestrpljiv is most naturally if you stay/remain impatient.

If Croatian wanted to emphasize become impatient, postaneš nestrpljiv would often be clearer.

Because after ostati, the adjective is usually in the nominative, agreeing with the subject.

Here the subject is the implied you, and nestrpljiv is the predicate adjective describing that subject:

• Ti ostaneš nestrpljiv. = You remain impatient.

So this is the normal pattern:

• ostati + adjective in nominative

Examples:

• Ostao je miran. = He remained calm.
• Ostala je ozbiljna. = She remained serious.

Čekanja is the genitive singular of čekanje, which is a verbal noun meaning waiting.

So:

• čekati = to wait
• čekanje = waiting
• čekanja = of waiting

That is why svaka minuta čekanja literally means every minute of waiting.

This is a very common Croatian structure:

• sat vremena = an hour of time
• čaša vode = a glass of water
• minuta čekanja = a minute of waiting

After a measure/unit word like minuta, the following noun is often in the genitive.

This is how Croatian writes the future when the auxiliary comes after the infinitive.

The verb is činiti. In the future:

• činit će
  not
• činiti će

When the enclitic future form of htjeti (ću, ćeš, će, ćemo, ćete, će) follows the infinitive, the final -i of the infinitive is dropped in writing:

• raditi → radit ću
• nositi → nosit će
• činiti → činit će

An alternative word order is also possible:

• Svaka minuta čekanja će ti se činiti dužom...

Both are correct; činit će is just the form used here.

The verb here is činiti se, which means to seem or to appear.

So:

• činiti se = to seem
• ti = to you

That means:

• činit će ti se = it will seem to you

Very literally, the sentence says something like:

• every minute of waiting will seem to you longer...

The se is part of the verb expression, while ti is a dative pronoun showing the experiencer.

Examples:

• To mi se čini čudno. = That seems strange to me.
• On ti se čini umoran. = He seems tired to you.

Because it is the instrumental singular feminine form of the comparative adjective, agreeing with minuta.

Here is the chain:

• base adjective: dug = long
• comparative: duži = longer
• feminine nominative: duža
• feminine instrumental: dužom

With činiti se, Croatian commonly uses the predicate adjective in the instrumental:

• minuta će ti se činiti dužom

Since minuta is feminine singular, the adjective matches it:

• dužom

So grammatically this means (as) longer.

Because the comparison is followed by a whole clause, not just a noun or adjective.

Here the comparison is:

• dužom nego što stvarno jest
• longer than it really is

The part after nego što is a clause:

• što stvarno jest = than it really is

A useful rule of thumb:

• use nego što when what follows is a clause
• use od more often when what follows is a noun phrase

Compare:

• Viši je nego što sam mislio. = He is taller than I thought.
• Viši je od brata. = He is taller than his brother.

Because je is a clitic form, while jest is the full stressed form of to be.

Croatian often uses jest when the verb stands in a position where the clitic je would sound wrong or cannot stand naturally on its own, especially near the end of a clause.

So:

• nego što stvarno jest = than it really is

Using jest here is natural and standard.

A simple way to think of it:

• je = short, unstressed form
• jest = full form, used when stress or position requires it

Yes. Croatian word order is fairly flexible, especially with clitics and future forms.

This sentence could be reshaped in other acceptable ways, for example:

• Svaka minuta čekanja će ti se činiti dužom nego što stvarno jest.
• Svaka će ti se minuta čekanja činiti dužom nego što stvarno jest.

The version you have:

• Svaka minuta čekanja činit će ti se dužom...

is perfectly normal and stylistically smooth.

The main things that stay the same are:

• svaka minuta čekanja is the subject
• činiti se means seem
• ti means to you
• dužom nego što stvarno jest gives the comparison