Dizel i benzin su goriva.

Because the subject is plural.

In Dizel i benzin, you have two things joined by i = and:

• dizel
• benzin

So Serbian treats the subject as plural: diesel and gasoline.
That means the verb to be must also be plural:

• je = is
• su = are

So:

• Dizel je gorivo. = Diesel is a fuel.
• Benzin je gorivo. = Gasoline is a fuel.
• Dizel i benzin su goriva. = Diesel and gasoline are fuels.

Because it refers to both dizel and benzin as two members of the category fuel.

The singular noun is:

• gorivo = fuel

Its plural is:

• goriva = fuels

So the sentence literally says:

• Diesel and gasoline are fuels

If you used singular gorivo, the sentence would sound different:

• Dizel i benzin su gorivo.

That can sometimes be understood in a more collective or category sense, but for a basic learner sentence, su goriva is the straightforward and expected form.

It is in the nominative plural.

In Serbian, when you use the verb biti (to be) in a sentence like X is Y or X are Y, the noun after it is usually also in the nominative:

• Ovo je kuća. = This is a house.
• To su automobili. = Those are cars.
• Dizel i benzin su goriva. = Diesel and gasoline are fuels.

So:

• gorivo = nominative singular
• goriva = nominative plural

Yes, both are normally treated as masculine singular nouns.

You can see that in sentences like:

• Dizel je skup. = Diesel is expensive.
• Benzin je skup. = Gasoline is expensive.

Even though both are masculine singular individually, when joined by i (and), they form a plural subject, so the verb becomes su.

Because Serbian does not use articles like English a/an/the.

So Serbian simply says:

• Dizel i benzin su goriva.

Depending on context, English might translate this as:

• Diesel and gasoline are fuels
• Diesel and gasoline are types of fuel
• Diesel and gasoline are fuels

Serbian leaves that article meaning to context.

Yes, Serbian word order is more flexible than English, although some versions sound more natural than others.

The neutral order here is:

• Dizel i benzin su goriva.

You could also say:

• Goriva su dizel i benzin.

That puts more focus on goriva or sounds a bit more like The fuels are diesel and gasoline / Diesel and gasoline are what count as fuels.

For a learner, the original order is the most natural basic pattern:

• subject + verb + complement

In most simple cases, yes.

Here:

• dizel i benzin = diesel and gasoline

I is the normal Serbian coordinating conjunction and.

Be careful with pronunciation, though: Serbian i is pronounced like the ee in see, not like English eye.

A simple approximate pronunciation is:

• Dizel → DEE-zel
• i → ee
• benzin → ben-ZEEN
• su → soo
• goriva → GO-ree-va

So the whole sentence is roughly:

• DEE-zel ee ben-ZEEN soo GO-ree-va

This is only an English-based approximation, but it is close enough to help a beginner.

In Serbian, nouns ending in -o or -e are very often neuter.

So:

• gorivo is a neuter noun

Its plural is:

• goriva

This is a common pattern:

• selo → sela
• more → mora
• gorivo → goriva

Yes, and that would mean something like:

• Diesel and gasoline are types of fuel

That version is a bit more explicit. It uses:

• vrste = types
• goriva = of fuel

But the original sentence:

• Dizel i benzin su goriva.

is simpler and perfectly natural for learning the basic structure.

You should include it.

In Serbian, the present-tense forms of biti (to be) are normally used in sentences like this:

• On je student. = He is a student.
• Oni su studenti. = They are students.
• Dizel i benzin su goriva.

So su is not optional here in standard Serbian.

You can make singular sentences by talking about each fuel separately:

• Dizel je gorivo. = Diesel is a fuel.
• Benzin je gorivo. = Gasoline is a fuel.

That helps show the pattern clearly:

• singular subject → je
• plural subject → su

and

• singular complement → gorivo
• plural complement → goriva