Learn Turkish: Mindere oturmadan önce rahatla. - Relax before sitting on the mat.

To say 'before doing something' in Turkish, you take the verb stem (otur-), add the ending -madan (or -meden, depending on vowel harmony), and follow it with the word önce. So, otur- + -madan + önce gives us 'before sitting'.

We translate this as 'sitting on the mat'. Shouldn't we use the locative ending -de instead of the dative -e in mindere?

In English, we say 'sit on something', but in Turkish, the action of sitting down is treated as a motion towards a destination. Therefore, oturmak takes the dative case (-e or -a) when you are talking about the act of taking a seat (mindere otur = sit onto the mat). If you want to say you are already sitting there, you would use the locative (minderde oturuyorum = I am sitting on the mat).

Could we drop the word önce from this sentence?

Yes, you often can! The suffix -madan / -meden already carries a meaning like 'without doing'. So Mindere oturmadan rahatla can mean 'Relax without sitting on the mat', which implies doing it beforehand. However, adding önce makes it specifically and clearly mean 'before'.

What form is the verb rahatla in?