Learn Turkish: Onu çocukluktan beri tanıyorum. - I have known him since childhood.

The word for childhood is çocukluk. To say "since" a certain point in time, Turkish uses the ablative case suffix (-dan/-den) followed by the postposition beri. So, çocukluk becomes çocukluktan, and adding beri gives us "since childhood".

Why does çocukluk take the suffix -tan instead of -dan?

This is because of consonant harmony. The word çocukluk ends in k, which is a voiceless (hard) consonant. Because of this, the d in the -dan suffix hardens into a t to match, making it -tan.

The English translation says "I have known", so why does the Turkish sentence use the present continuous tense tanıyorum?

In English, we use the present perfect ("have known") for a state that started in the past and continues today. But in Turkish, if the state is still ongoing right now, you use the present continuous tense (-iyor). Since you still know the person today, you use tanıyorum.

Could I use biliyorum instead of tanıyorum to say "I know him"?