Hard adjective - Requiring considerable physical or mental effort.
Usage example: a hard exam to pass
Stiff is a synonym for hard in firm topic. In some cases you can use "Stiff" instead the word "Hard" as an adjective or a noun, when it comes to topics like tough, unrelenting, alcoholic, rocklike. popular alternative
Stiff adjective - Incapable of or highly resistant to bending.
Usage example: use a stiff piece of paper for the project
Hard is a synonym for stiff in difficult topic. You can use "Hard" instead an adjective "Stiff", if it concerns topics such as severe, property, inflexible, rigid. popular alternative
Both words in one sentence
- Not What It Looks Like: Hearing Rise talk about how stiff and hard Masaya is, Tamie bursts into the room fully expecting her to be giving him a handjob.
Source: Visual Novel / Princess Evangile
- The blade had a thick spine so that it wouldn't flex easily and was often of a hard temper so that it could be both narrow and stiff.
- And the competition for your job wasn't exactly stiff.Brendan: I find that hard to believe.Al: *shocked silence* Also:Brendan: You can sleep in my bed.
Source: Webcomic / O Human Star
Google Ngram Viewer shows how "hard" and "stiff" have occurred on timeline