Learn Italian: Dieci anni fa, un gadget simile era difficilmente immaginabile. - Ten years ago, a similar gadget was hardly imaginable.

You drop the final -re from the infinitive immaginare (to imagine) and add the suffix -bile. Because it's an -are verb, the vowel stays an 'a', creating -abile. Verbs ending in -ere or -ire usually take -ibile instead (like credere becoming credibile). It works exactly like the English "-able" or "-ible", meaning "capable of being imagined".

The translation says "hardly". Can I use difficilmente anytime I want to say "hardly" in English?

Not quite. Difficilmente literally means "with difficulty" or "unlikely". Here, difficilmente immaginabile translates nicely to "hardly imaginable" because it describes the high level of difficulty in imagining it. However, if "hardly" means "barely" in English (e.g., "I hardly slept"), you cannot use difficilmente. You would instead use words like appena or a stento.

Why use un gadget simile instead of just saying un gadget come questo?

While un gadget come questo (a gadget like this) is grammatically correct, un gadget simile (a similar gadget / such a gadget) sounds much more natural and elegant in Italian when referring to a broad category of things. Also, if you place the adjective before the noun (un simile gadget), it adds a bit more dramatic emphasis, closer to "such a gadget!".