Learn Arabic (Modern Standard): هؤلاء عاملون مجتهدون.

In Arabic, plurals are treated differently depending on whether they are human or non-human. Because عاملون (workers) refers to human beings, we use the true plural demonstrative هؤلاء. If we were talking about non-human objects, like "these computers", we would treat them grammatically as a single female entity and use هذه.

How do we know this means "These are hardworking workers" and not just the phrase "These hardworking workers..."?

The invisible "are" is created by a difference in definiteness. هؤلاء (These) is naturally a definite word, but عاملون (workers) is indefinite (it has no الـ at the beginning). In Arabic, when a definite subject is followed by an indefinite noun, it forms a complete sentence with "is" or "are". If it had said هؤلاء العاملون المجتهدون, it would just be an incomplete phrase meaning "These hardworking workers...".

Why do both عاملون and مجتهدون end with the ـون (-uun) suffix?

They end in ـون because they are "sound masculine plurals" in the nominative case. عاملون is the predicate of the sentence (the core news about the subject), which puts it in the nominative case. The adjective مجتهدون (hardworking) must match the noun it describes in gender, number, definiteness, and case, so it copies the exact same ـون ending.

Why doesn't هؤلاء have an alif (ا) after the first letter, even though it's pronounced with a long "a" sound?