Izvođač radova i podizvođač počinju sutra.

This is a common business/construction phrase.

• izvođač = performer, executor, contractor
• radova = of works

So izvođač radova is best understood as contractor or the company/person carrying out the works.

Literally, it is something like executor of works, but in natural English contractor is usually the best translation.

Radova is the genitive plural of radovi.

Croatian often uses the genitive after a noun to express an of ... relationship:

• izvođač radova = contractor of works
• voditelj projekta = manager of the project
• plan gradnje = plan of construction

So radova is there because izvođač is being specified: a contractor of what? Of works.

In Croatian, radovi very often means works in the construction/technical sense. This is similar to English expressions like construction works, road works, or public works.

So izvođač radova is a fixed and very natural phrase in administrative and construction language. Even if English might sometimes prefer contractor, Croatian commonly keeps radovi in the plural.

Podizvođač means subcontractor.

It is built from:

• izvođač = contractor
• pod- = sub-, under-

So:

• izvođač radova = contractor
• podizvođač = subcontractor

This is a very common pair in construction and legal/business contexts.

Because the subject has two parts:

• izvođač radova
• i podizvođač

Since they are joined by i = and, the verb must be plural.

So:

• Izvođač radova počinje sutra. = The contractor starts tomorrow.
• Izvođač radova i podizvođač počinju sutra. = The contractor and the subcontractor start tomorrow.

This is very normal in Croatian.

A present-tense form can be used for a planned, scheduled, or near-future action, especially when there is a time word like sutra = tomorrow.

So počinju sutra naturally means:

• they start tomorrow
• they are starting tomorrow

English does something similar in sentences like The meeting starts tomorrow.

Yes. They are both in the nominative singular because together they form the subject of the sentence.

• izvođač = nominative singular
• podizvođač = nominative singular

Even though the whole subject is plural in meaning because there are two nouns joined by i, each individual noun stays singular.

Croatian does not have articles like English a/an and the.

So Izvođač radova i podizvođač počinju sutra could mean:

• The contractor and the subcontractor start tomorrow
• sometimes, depending on context, A contractor and a subcontractor start tomorrow

Usually the context tells you whether the people/entities are specific or not.

Yes. Croatian word order is more flexible than English word order.

This sentence could also appear as:

• Sutra počinju izvođač radova i podizvođač.
• Izvođač radova i podizvođač sutra počinju.

All of these are grammatical. The difference is mainly emphasis:

• ... počinju sutra puts tomorrow at the end, which can sound a bit more informational or emphasized.
• Sutra počinju ... puts the time first.

A few sounds may need attention:

• č in izvođač and podizvođač: like ch in church, but usually a bit firmer
• đ in izvođač and podizvođač: roughly like the j in jam, but softer
• nj in počinju: like the ny sound in canyon

A rough pronunciation guide:

• izvođač ≈ eez-VO-jach
• podizvođač ≈ po-deez-VO-jach
• počinju ≈ po-CHEEN-yoo
• sutra ≈ SOO-tra

These are only approximations, but they help with the main sounds.