"issue" when you mean "problem"
"disturbed" when you mean "psychotic"
"sanitary engineer" when you mean "janitor"
"unusual" when you mean "bizarre"
"ample" or "horizontally endowed" when you mean "fat"
"challenged" when you mean "crippled" or "retarded" or "short" or "clueless"
"differently abled" when you mean "disabled"
"ill" when you mean "drunk"
"vertically challenged" when you mean "midget"
The key is that the words themselves are not inherently a euphemism. It's their use in place of something else that you don't want to say that makes them euphemistic. A euphemistic expression can have a perfectly normal, literal, unquestionable meaning. It becomes a euphemism when you are using it as a "nicer" way of saying something.
A euphemism for fat is "pleasingly plump". A euphemism for feeling sick is feeling "under the weather". A euphemism for being fired is being "laid off".
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Chairperson
Examples of nouns that do not change form for singular to plural:advicedeerbinocularsmeansoffspringpantssalmonscissorssheepspeciesswinetrout
If by 'invisible' you mean abstract nouns, words for things that can't be seen, heard, smelled, tasted, or touched, some examples are:attitudebeautycaredestinyeducationfriendshipgratitudehopeideajokekindnesslovemysteryneedopinionpeacequestreasonseasontrusturgevalueworryyearzeal
A euphemism for fat is "pleasingly plump". A euphemism for feeling sick is feeling "under the weather". A euphemism for being fired is being "laid off".
Some examples of transitional words used to contrast statements are:butoryetstillratherthoughinsteadhoweverotherwiseconverselyalternativelynonethelessneverthelessnotwithstanding
Examples of one syllable words are:aanareboarboreboxcancarcaredodogdoordougheatedgeeighteyefiveforfourgivegogoathihighheightinisitjayjokejoykeepkingkneelatelightlotmemoonmynonotnoteofoneownpoorpotpourquestquitquiteratrestrootroutesinsomesongthrowtotootoottwousevieviewwhatwhenwhoyearyetyouzipzonezoo
Euclid, euclidean, Europe, European, euphemism, euphemistic, euphoria, euphoric, euthanasia, Eustachian tube
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Euphemistic
You will find some examples in the Wikipedia article on "Polish language".
Some examples of words with the suffix -sion are discussion, decision, and expression.
Some examples of words with the suffix "-an" include human, urban, and ocean.
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Chairperson
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