Upitnik za davaoca nije težak.

It breaks down like this:

• Upitnik = the subject, questionnaire
• za davaoca = a phrase meaning for the donor
• nije = is not
• težak = an adjective meaning difficult or heavy, depending on context

So the structure is:

subject + prepositional phrase + verb + predicate adjective

In very literal English word order, it is:

Questionnaire for donor is not difficult.

Because Serbian often expresses intended for / meant for someone with za + accusative.

So:

• upitnik za davaoca = a questionnaire for the donor
• literally, questionnaire for donor

This is very natural Serbian. English often puts nouns together, like donor questionnaire, but Serbian more often uses a prepositional phrase:

• upitnik za davaoca
• lek za decu = medicine for children
• poklon za prijatelja = gift for a friend

Because za in the sense of for takes the accusative case.

The dictionary form is davalac = donor.

But here the noun is after za, so it must be in the accusative:

• davalac = nominative
• davaoca = accusative singular

Also, this is an animate masculine noun, and those often have an accusative form that looks like the genitive. So davaoca is exactly what you would expect here.

It is in the nominative singular, because it is the subject of the sentence.

The sentence is about the questionnaire, so upitnik stays in the nominative:

• Upitnik ... nije težak.

That is also why the adjective težak matches it in masculine singular nominative.

Because adjectives in Serbian agree with the noun they describe.

Here the noun is upitnik, which is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives:

• težak = masculine singular
• teška = feminine singular
• teško = neuter singular

So:

• Upitnik nije težak. = correct
• Knjiga nije teška. = the book is not difficult/heavy
• Pitanje nije teško. = the question is not difficult

It can mean both, depending on context.

With upitnik (questionnaire), the natural meaning is usually difficult, not physically heavy.

So in this sentence, nije težak is best understood as:

• is not difficult
• is not hard

But literally, težak is the ordinary word for heavy, and Serbian often uses the same adjective for both physical and figurative difficulty.

Because this is the standard negated form of je (is) from the verb biti (to be).

In standard Serbian, these negative present-tense forms are written together:

• nisam = I am not
• nisi = you are not
• nije = he/she/it is not
• nismo = we are not
• niste = you are not
• nisu = they are not

So nije is just the normal form, not a special spelling for this sentence.

Because Serbian has no articles.

That means upitnik can mean:

• a questionnaire
• the questionnaire

The exact English article depends on context, not on a separate Serbian word.

If Serbian speakers want to be more specific, they may use other words, for example:

• ovaj upitnik = this questionnaire
• jedan upitnik = a questionnaire / one questionnaire

But in a normal sentence like this, no article is needed.

Yes, Serbian word order is fairly flexible.

Upitnik za davaoca nije težak is a neutral, natural order.

But you could also move parts around for emphasis, for example:

• Za davaoca upitnik nije težak.
  = As for the donor, the questionnaire is not difficult.

• Upitnik nije težak za davaoca.
  This can also work, but it puts more emphasis on for the donor.

So the original sentence is probably the most neutral version, but other orders are possible.

A rough pronunciation guide is:

u-pit-nik za da-va-o-tsa ni-ye te-zhak

A few useful sound notes:

• j sounds like English y
• ž sounds like the s in measure
• c sounds like ts

So:

• nije sounds roughly like nee-ye
• težak sounds roughly like te-zhak
• davaoca ends with -tsa

Yes, it can.

Upitnik can mean:

• question mark
• questionnaire

Context tells you which meaning is intended.

In Upitnik za davaoca nije težak, it clearly means questionnaire, because a question mark for the donor would not make sense here.