If they're not commonly used, write them in italics:

<aside> ✅ aegis

</aside>

<aside> ✅ bactoraemia

</aside>

<aside> ✅ diaeresis

</aside>

Just use “e” or “ae” spellings, not "æ":

<aside> ✅ archaeology

</aside>

<aside> ✅ encyclopaedia

</aside>

<aside> ✅ haematology

</aside>

Use plural forms if they're commonly used:

<aside> ✅

</aside>

<aside> ✅ basis/bases

</aside>

<aside> ✅ crisis/crises

</aside>

<aside> ✅ millennium/millennia

</aside>

<aside> ✅ alumnus/alumni

</aside>

But avoid using "alumnus" and "alumni" wherever possible – see Alumni, alumna, alumnus to find out why.

And be wary of words with more than one plural form with different meanings:

They may have found the formulas for a long and happy life, but we're still looking for the maths and chemistry formulae.