Learn Finnish: Silloin me asuimme Porvoossa. - Back then we lived in Porvoo.

asuimme is the past tense (often called the imperfect) of the verb asua (to live). To form it, you take the stem asu-, add the past tense marker -i-, and finally add the 'we' personal ending -mme. asumme is the present tense, meaning 'we live'.

Why use silloin here instead of sitten?

silloin means 'then' or 'at that time' when referring to a specific period or moment in the past, just like 'back then' in this sentence. sitten is usually used for a sequence of events, meaning 'then' or 'next' (like 'first we ate, then we left').

Why does Porvoo take the -ssa ending instead of -ssä?

The ending for 'in' (the inessive case) changes based on vowel harmony. Because the name Porvoo contains the back vowel 'o' and no front vowels (like 'ä', 'ö', or 'y'), it requires the back vowel ending -ssa.

Do you have to include the pronoun me here?

No, it is optional. Because the verb ending -mme already tells us the subject is 'we', you could perfectly well say Silloin asuimme Porvoossa. Including me often just adds a little bit of emphasis or sounds slightly more conversational.