In the past English had a fascinating system of verb conjugations that it inherited from its ancestor languages. We'll look at one word in particular - a plural form of "to be", so the modern "are". Technically, it's a third-person plural present indicative as wiktionary says.
Quoting wiktionary about Middle English:
The usual plural form of been is aren in the North, been in the Midlands, and beth in the South; sind also existed, especially early on, but was not the predominant form in any area.
And if we look at the page for "are":
reinforced by Old Norse plural forms in er-
So there were a few variants for this word, and only one really survived into modern English. But I think "sind" is particularly curious - because it has cognates in some interesting places.
You might've heard a Latin phrase "Ubi sunt" - literally "where are... [they]". This "sunt" is a plural of Latin word for "to be", and is cognate with "sind".
If we look at Proto-Slavic, we can find this word:
*sǫtь
third-person plural present of *byti
...which resulted in Russian "суть" (sut') and Polish "są".
It's also a relative or our "sind". In Proto-Slavic there formed nasal vowels where some other languages preserved a 'n' next to a vowel instead, and if we look at the full etymology of these two:
sind: From Proto-Germanic *sindi, third-person plural present indicative of *wesaną, from Proto-Indo-European *h₁sénti, third-person plural present indicative of *h₁ésti.
*sǫtь: From Proto-Balto-Slavic *sánti, from Proto-Indo-European *h₁sonti, verb form of *h₁ésti.
It's really, really interesting to me to take a random word and look into it as much as I can, and I always find unexpected cognates and interesting insight into how words originate and how languages evolve in general.