Learn Portuguese (Portugal): Os engenheiros devem ajudar os operários.

When using modal verbs like dever (must/should) or poder (can) to express an obligation or ability, the second verb follows immediately in its infinitive form (ajudar). You don't need to add a linking preposition like a or de between them.

Could I use têm de instead of devem here?

You could, but it changes the tone slightly. Devem ajudar means "must help" or "should help" in the sense of a strong moral duty or expectation. Têm de ajudar means "have to help" and sounds more like a strict, inescapable rule.

What is the difference between operários and trabalhadores?

Trabalhadores is a general word for "workers" in any field or profession. Operários specifically refers to manual laborers, factory workers, or construction workers. Since this sentence is about engineers, it makes sense that they are helping the manual operários on their site.

If I wanted to say "Engineers must help workers" in general (without "The"), would I remove the word Os?