Learn Turkish: Üniversiteden beri arkadaşız. - We have been friends since university.

The postposition beri (since) requires the noun before it to take the ablative case ending, which is -den or -dan (meaning 'from'). You will always use this 'from... since' pattern for a starting point in time.

The English translation says 'we have been friends', but arkadaşız just means 'we are friends'. Why isn't a past tense used here?

In Turkish, if a situation started in the past and is still true right now, you use the present tense with beri. Since you are still friends today, you just say arkadaşız (we are friends). English requires the present perfect ('have been'), but Turkish uses the present tense.

How does the word arkadaşız break down?

It is simply arkadaş (friend) plus the first-person plural suffix -ız (we are). In Turkish, you don't need a separate verb for 'to be' in the present tense; you just attach the person ending directly to the noun or adjective.