Final answer:
The most difficult vocabulary words to use or understand in English are 'ubiquitous,' 'idiosyncrasy,' and 'ephemeral' as they all have unique meanings and usage contexts.
Step-by-step explanation:
The most difficult vocabulary word to use or understand can vary from person to person. However, a commonly challenging word for many students is 'ubiquitous.' Ubiquitous means being present everywhere or being widespread. For example, we can say that smartphones are ubiquitous in today's society because almost everyone owns one.
Another challenging word is 'idiosyncrasy.' This word refers to a peculiar behavioral characteristic or habit that is unique to an individual or a group. For instance, someone might have the idiosyncrasy of always double-checking the lock before leaving their house.
Lastly, 'ephemeral' is a word that often gives students difficulty. Ephemeral means lasting for a short period of time or being temporary. For example, a rainbow is ephemeral because it only appears briefly after rainfall.
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