Ovaj projekat nije lak, ali pokušaću da ga završim večeras.

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

• projekat is masculine singular
• the phrase is in the nominative because it is the subject of the sentence

So the correct form is:

• ovaj projekat = this project

Compare:

• ovaj = masculine singular
• ova = feminine singular
• ovo = neuter singular

Here projekat is the subject of the sentence, so it appears in the nominative singular.

The sentence is basically:

• Ovaj projekat nije lak = This project is not easy

If the noun had a different role in the sentence, its form could change because Serbian uses cases. For example, later in the sentence, the project is referred to by ga, which is the accusative object pronoun.

In Serbian, the negative form of je/sam-type forms of biti (to be) is built as a single word:

• nije = is not
• nisam = I am not
• nismo = we are not, etc.

So:

• nije lak = is not easy

You do not normally say ne je.

Because lak is an adjective describing the masculine noun projekat.

• lak = masculine singular
• laka = feminine singular
• lako = neuter singular, and also often an adverb meaning easily

Since projekat is masculine, you need:

• projekat je lak = the project is easy

If you said lako, it would not match the noun correctly here.

Yes, you could, but lak is the more natural choice for easy in this sentence.

• lak often means easy, not difficult, or light
• lagan often means light in weight or not heavy, and sometimes gentle/light in a broader sense

For a task, assignment, exam, or project, lak is usually the normal word for easy.

So:

• nije lak = the most natural choice here

Pokušaću is the 1st person singular future tense of pokušati (to try).

It means:

• I will try

In Serbian, future tense can be formed with ću plus the infinitive, and when the verb comes before ću, they are often written together:

• Ja ću pokušati
• Pokušaću

These mean the same thing.

With pokušati, the -ti drops before ću:

• pokušati → pokušaću

Because Serbian often omits subject pronouns when the verb ending already makes the subject clear.

• pokušaću already means I will try

So ja is not necessary unless you want emphasis or contrast.

For example:

• Pokušaću da ga završim. = neutral, natural
• Ja ću pokušati da ga završim. = I will try, perhaps contrasting with someone else

This is very common in Serbian.

Because Serbian very often uses da + present tense where English uses an infinitive.

So:

• pokušaću da završim literally looks like I will try that I finish
• but it really means I will try to finish

This structure is extremely common in Serbian, especially in everyday language.

A learner should get used to:

• želim da idem = I want to go
• moram da radim = I have to work
• pokušaću da završim = I will try to finish

Because after da, Serbian uses a present-tense verb form, not the infinitive.

So:

• da završim = to finish / that I finish

Here završim is the 1st person singular present of završiti.

Also, završiti is perfective, which fits well because the speaker is talking about completing the project by tonight.

Compare:

• završiti / završim = to finish, to complete
• završavati / završavam = to be finishing, to finish habitually or over time

In this sentence, the idea is one completed action, so završim is the right choice.

Ga is a clitic pronoun meaning him/it, and here it means it.

It refers back to:

• projekat

So:

• da ga završim = to finish it

Because projekat is a masculine singular noun, the pronoun used here is ga.

Because short object pronouns like ga are clitics, and clitics have special placement rules in Serbian.

In a clause like this, the normal order is:

• da ga završim

not:

• da završim ga

So the pronoun usually comes before the verb in this kind of da-clause.

This is something learners simply need to get used to, because Serbian clitic placement does not work like English word order.

Yes, Serbian word order is fairly flexible, so večeras can move depending on emphasis.

The original sentence:

• Ovaj projekat nije lak, ali pokušaću da ga završim večeras.

is very natural and neutral.

But you could also say:

• Večeras ću pokušati da ga završim.
• Pokušaću večeras da ga završim.

These all mean roughly the same thing, but the emphasis changes slightly.

• final večeras sounds natural and straightforward
• sentence-initial večeras puts more focus on tonight

A few useful pronunciation points:

• š sounds like sh in ship
• č sounds like ch in church
• ć is a softer sound than č; English does not have an exact match, but it is somewhat like a softer ch/t
• j sounds like English y in yes

So roughly:

• pokušaću ≈ po-ku-SHA-chyu (very approximate)
• večeras ≈ VE-che-ras

Also remember that Serbian spelling is very phonetic, so words are usually pronounced much as they are written.

It has two parts joined by ali (but):

1. Ovaj projekat nije lak
   = This project is not easy

2. ali pokušaću da ga završim večeras
   = but I will try to finish it tonight

So the sentence pattern is:

• statement 1
• ali
• contrasting statement 2

This is a very common and useful Serbian sentence structure.