Learn Turkish: Yoga yapmadan önce su içiyorum. - Before doing yoga, I drink water.

The suffix -mAdAn önce follows A-type vowel harmony. Because the vowel in the verb root yap is a back vowel (a), the suffix becomes -madan önce. If the verb root had a front vowel, like git (go), it would be gitmeden önce.

Why doesn't yapmadan önce have a personal ending? How do we know who is doing the yoga?

The -mAdAn önce structure is completely fixed and never takes personal endings. The listener knows who is doing the yoga by looking at the main verb at the end of the sentence. Since it ends with -um (içiyorum, meaning "I drink"), we know "I" am the one doing both actions.

The English translation says "I drink" (a routine), but içiyorum uses the continuous tense. Why use it here?

While -iyor is the present continuous tense (usually "am drinking"), Turkish speakers very frequently use it for habits, routines, and general facts too. You will often see -iyor used where English would use the simple present tense.

Why is it just su and not suyu?

You use the bare form su because you are talking about drinking water as a general activity, not a specific, previously mentioned glass of water. When an object is general and non-specific like this, it doesn't get an ending and sits directly in front of the verb.