Ovaj projekat je teži nego onaj.

Ovaj means this. It is the masculine singular form of the demonstrative pronoun/adjective used for something close to the speaker.

In ovaj projekat, it agrees with projekat because projekat is:

• masculine
• singular
• in the nominative case

So:

• ovaj projekat = this project

If the noun were feminine or neuter, the form would change:

• ova knjiga = this book
• ovo pitanje = this question

Projekat is the nominative singular form, which is the form typically used for the subject of the sentence.

In this sentence, ovaj projekat is the subject:

• Ovaj projekat je teži... = This project is harder...

Serbian changes noun forms depending on case, number, and sometimes role in the sentence. But here the basic dictionary form is needed.

You may also see projekt in some usage, but projekat is a standard Serbian form.

Je means is. It is the 3rd person singular form of the verb biti (to be).

So:

• ovaj projekat je teži = this project is harder

Some present-tense forms of biti are:

• sam = am
• si = are
• je = is
• smo = are
• ste = are
• su = are

Teži is the comparative form of the adjective težak.

• težak = heavy / difficult / hard
• teži = heavier / more difficult / harder

In this sentence, it means harder / more difficult, not physically heavier.

So:

• Ovaj projekat je težak. = This project is difficult.
• Ovaj projekat je teži nego onaj. = This project is harder than that one.

This is because Serbian comparatives are often formed in ways that are not fully regular from an English speaker’s point of view.

For težak, the comparative is:

• težak → teži

This is a standard comparative pattern for some adjectives, where the stem changes a bit.

Other examples:

• lak → lakši = easy/light → easier/lighter
• star → stariji = old → older
• lep → lepši = beautiful → more beautiful

So teži is something you should learn as the comparative form of težak.

Here, teži is the masculine singular nominative form, matching projekat.

Adjectives in Serbian agree with the noun they describe. Since projekat is masculine singular nominative, the adjective must also be masculine singular nominative:

• ovaj projekat je teži

If the noun changed, the adjective form would change too:

• Ova knjiga je teža. = This book is harder.
• Ovo pitanje je teže. = This question is harder.

So the ending reflects agreement with the noun.

Nego means than in comparisons.

So:

• teži nego onaj = harder than that one

This is the standard way to compare two things:

• veći nego... = bigger than...
• brži nego... = faster than...
• skuplji nego... = more expensive than...

In this sentence, nego introduces the second item in the comparison.

Yes, Serbian can often use od for comparisons:

• Ovaj projekat je teži od onog.

This also means This project is harder than that one.

The difference is grammatical:

• with nego, you get onaj
• with od, you use the genitive form: onog

So:

• teži nego onaj
• teži od onog

Both are natural, but the form after them changes.

Because the sentence uses nego, not od.

With nego, the compared word often stays in the same basic form you would expect for an implied noun:

• nego onaj = than that one

Here onaj stands for something like:

• onaj projekat = that project

The noun projekat is omitted because it would be repetitive.

But if you use od, the following word must go into the genitive:

• od onog = than that one

So:

• nego onaj ✔
• od onog ✔
• nego onog ✘ in this structure
• od onaj ✘

Onaj means that one or that. It refers to a masculine singular noun that is understood from context.

Here it stands for:

• onaj projekat = that project

Since projekat was already mentioned, Serbian leaves it out:

• Ovaj projekat je teži nego onaj. Literally: This project is harder than that one.

This is very common in Serbian, just as in English.

Because it is unnecessary. Once the noun is clear, Serbian often uses a demonstrative alone:

• onaj = that one

So instead of saying:

• Ovaj projekat je teži nego onaj projekat

Serbian normally says:

• Ovaj projekat je teži nego onaj.

English does the same:

• This project is harder than that one.

The given word order is the most neutral and natural:

• Ovaj projekat je teži nego onaj.

Because Serbian has case marking and agreement, word order is somewhat flexible, but not all versions sound equally natural.

You might move words for emphasis, but the standard learning pattern is:

• subject + je + comparative + nego + second item

So this sentence is a very good model to remember.

It can mean both, depending on context.

• physical: Ovaj kofer je težak. = This suitcase is heavy.
• figurative/intellectual: Ovaj projekat je težak. = This project is difficult.

In your sentence, because the noun is projekat, the meaning is clearly difficult / hard, not heavy.

So teži here means:

• harder
• more difficult

You would change the demonstratives, the verb, and the adjective to plural:

• Ovi projekti su teži nego oni.

Breakdown:

• ovi = these
• projekti = projects
• su = are
• teži = harder (masculine plural)
• oni = those

So the sentence follows the same pattern, just with plural agreement.