倔强 vs 固执 vs 顽固 usage
倔 = blunt 强 = strong
倔强 =  stubborn (resist with strong will, e.g. force, persuasion)

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固 = fixed
执 = holding on
固执 = stubborn (fixated and holding on to it, e.g. opinion, believe)

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顽 = tough
固 =  fixed
顽固 = stubborn  (stubbornly fixated , e.g. opinion, believe)

倔强, 固执 and 顽固 can all be a description of someone's stubborn personality.

倔强 emphasize on the strong-willed aspect of stubborn

固执 emphasize on the fixation aspect of stubborn

顽固 emphasize on the unyielding aspect of stubborn


You try to force or persuade your daughter to give up an unrealistic dream, but she stubbornly refuses. That is she being 倔强 (she is so strong-willed, she wouldn't yield to force or persuaded by reasons)
You try to convince an old man that he should not trust traditional Chinese medicine over modern western medicine, but he stubbornly holding on to the believe that traditional Chinese medicine works better than modern western medicine. That is he being 固执 or 顽固 (he is fixated on his believe and firmly holding on it)

固执 is a neutral term, we have a saying 擇善固执 (pick a good believe and hold firm), so when  we say someone being 固执 on a subject, we do not imply he is actually right or wrong

顽固 is mostly a derogatory term, when we say someone being 顽固 on a subject, we actually imply he is wrong