Learn Turkish: Sabahtan beri çalışıyorum. - I have been working since morning.

When using beri to mean "since", the time word it refers to must always take the ablative case (the "from" ending: -dan or -den). You can think of it literally as "from morning since". Without this ending, the phrase is grammatically incomplete.

Why does the ablative ending become -tan instead of the usual -dan here?

This is a quick reminder of consonant harmony. Because sabah ends in the voiceless consonant h, the d in the -dan ending hardens into a t to match it, giving us sabahtan.

The English translation uses "have been working". Why does Turkish just use the present continuous verb çalışıyorum ("am working")?

In Turkish, if an action started in the past and is still continuing right now, you simply use the present continuous tense (-ıyor). Unlike English, Turkish doesn't use a separate "have been" tense for this. The time phrase sabahtan beri already makes it perfectly clear that the action has been going on for a while.