Learn Hardcore Portuguese (Portugal): Viagem é divertida.

It means "Travel is fun" or, more literally, "Trip is fun." The sentence expresses the idea that the experience of traveling (or taking a trip) is enjoyable.

Why is the adjective divertida in the feminine form?

In Portuguese, every noun has a gender. The noun viagem is feminine, so the adjective that describes it must also be in the feminine form, which is why divertida (ending with an -a) is used instead of divertido.

I notice there’s no article before viagem. Is omitting the article correct here, and when should I use one?

Yes, in this context the article is omitted to give the sentence a general or proverbial tone. While you might more commonly say A viagem é divertida when referring to a specific or generic trip, omitting the article (as in Viagem é divertida) can be used for broad, general statements about the nature of travel.

Can I also express the idea using the infinitive form, like Viajar é divertido?

Absolutely. Viajar is the infinitive form of the verb, meaning "to travel." When you say Viajar é divertido, you emphasize the act of traveling, which translates to "Traveling is fun." The original sentence (Viagem é divertida) uses a noun, focusing on the concept or experience of a trip.