Learn Spanish (Latin America): ¡Qué bueno que la linterna ilumine la cavidad! - How good that the flashlight illuminates the cavity!

The phrase ¡Qué bueno que...! expresses a subjective opinion or evaluation. In Spanish, whenever you use a structure like this to express how you feel about a specific action, the verb that follows must be in the subjunctive.

Would it ever be correct to say ¡Qué buena que...! to match the feminine noun la linterna?

No, you must use bueno. In this structure, bueno isn't describing the flashlight; it's part of the impersonal expression Qué bueno (How good it is). Since it doesn't modify a specific noun, it stays in its default masculine or neuter form.

Could I use an infinitive and say ¡Qué bueno iluminar la cavidad!?

You can only use the infinitive if you are making a general statement without a specific subject doing the action. Because this sentence specifies that the flashlight (la linterna) is doing the illuminating, you have to use que followed by a conjugated subjunctive verb.

Does linterna translate to lantern in English?

It most commonly means flashlight. If you want to talk about a lantern, such as a camping lantern or a decorative street lamp, you would typically use the word farol or lámpara.