Ovaj posao traži puno strpljenja.

Because ovaj must agree with posao in gender, number, and case.

• posao = a masculine singular noun
• here it is the subject of the sentence, so it is in the nominative
• the masculine singular nominative form of this is ovaj

So:

• ovaj posao = this job

Other forms would appear in other cases, for example:

• ovog posla = of this job
• o ovom poslu = about this job

Posao is in the nominative singular.

In this sentence, ovaj posao is the subject:

• Ovaj posao traži puno strpljenja.

A useful way to think about it is:

• What requires a lot of patience?
  This job does.

That makes ovaj posao the subject, so nominative is used.

That is a very common question. Tražiti can mean several related things, including:

• to look for
• to ask for
• to demand
• to require

In this sentence, the meaning is require or call for:

• Ovaj posao traži puno strpljenja.
  = This job requires a lot of patience.

So the verb keeps the basic idea of asking for / demanding, and in English the most natural translation here is requires.

Traži is the:

• 3rd person singular
• present tense
• of the verb tražiti

Why 3rd person singular?

Because the subject is ovaj posao = this job, which is singular.

A quick mini-paradigm:

• ja tražim = I look for / ask for
• ti tražiš = you look for / ask for
• on/ona/ono traži = he/she/it looks for / asks for
• ovaj posao traži = this job requires

Because after puno meaning a lot of, Croatian normally uses the genitive.

So:

• puno
  • genitive

Here the noun is:

• strpljenje = patience

Its genitive singular form is:

• strpljenja

So:

• puno strpljenja = a lot of patience

This is a very common pattern in Croatian:

• puno vremena = a lot of time
• puno posla = a lot of work
• puno novca = a lot of money

It is genitive singular.

The base form is:

• strpljenje = patience

Because it comes after puno, it changes to the genitive:

• puno strpljenja

This is one of the most useful Croatian patterns to remember:
quantities often trigger the genitive.

Because strpljenje is an uncountable noun in this meaning, like patience in English.

You do not normally count patiences one by one. So Croatian treats it as a mass noun here:

• puno strpljenja = a lot of patience

This is similar to:

• puno vode = a lot of water
• puno vremena = a lot of time

So even though puno means a large amount, the noun itself can still be singular if it is uncountable.

In this sentence, puno is best understood as a quantity word meaning a lot of / much.

It does not behave exactly like a normal descriptive adjective such as:

• dobar posao = a good job
• težak posao = a hard job

Instead, puno introduces an amount:

• puno strpljenja = a lot of patience

For learners, the most useful practical rule is simply:

• after puno, use the genitive

Yes, absolutely.

• tražiti can mean to require
• zahtijevati also means to require / demand

So both are possible:

• Ovaj posao traži puno strpljenja.
• Ovaj posao zahtijeva puno strpljenja.

The version with zahtijeva may sound a bit more formal or explicit, while traži can sound a bit more everyday and idiomatic in this kind of sentence.

Because the sentence expresses a general fact or general characteristic.

• Ovaj posao traži puno strpljenja. = This job requires a lot of patience.

Croatian, like English, often uses the present tense for general truths and typical situations.

It does not mean only right now, at this moment. It means something like:

• this is generally true of this kind of job
• patience is normally needed for this job

Yes. Croatian word order is more flexible than English word order, although the default version here is very natural:

• Ovaj posao traži puno strpljenja.

You could also say:

• Puno strpljenja traži ovaj posao.

That version puts more emphasis on puno strpljenja.

Even though word order can change, the original order is the most neutral and easiest for learners to use.

Because Croatian has no articles.

English says:

• this job
• a job
• the job

Croatian does not have separate words for a/an/the. Instead, meaning is understood from context, word order, and demonstratives such as:

• ovaj = this
• taj = that
• onaj = that over there

So:

• ovaj posao already clearly means this job
• you do not need an article before posao

A rough English-friendly guide:

• traži ≈ TRAH-zhee
• strpljenja ≈ STRP-lyeh-nya

A few useful points:

• ž sounds like the s in measure
• nj sounds like the ny in canyon
• Croatian r can be strongly pronounced, even in clusters like strp-

The cluster in strpljenja is hard for English speakers at first, so it helps to say it slowly:

• str-plje-nja

Then speed it up gradually.