Learn Turkish: Günde bir kez kahve içiyorum.

To express 'per day' or 'a day' (like in 'once a day'), Turkish uses the locative suffix -de or -da ('in' or 'at'). So, günde literally translates to 'in the day'. You can use this pattern for other time words too, like haftada (a week / per week).

What exactly does kez mean in bir kez?

Kez means 'time' or 'instance'. Combined with bir (one), bir kez means 'one time' or 'once'. You might also hear bir defa or bir kere—they are all common ways to say the exact same thing.

Why use the continuous tense içiyorum ('I am drinking') for a daily habit?

In Turkish, the present continuous tense (with the -yor suffix) is very commonly used for general habits and routines, not just actions happening right this second. So içiyorum naturally translates to both 'I am drinking' and 'I drink'.

Why doesn't kahve have a specific object suffix (like kahveyi)?

We leave kahve in its bare dictionary form because it is a general, indefinite object here. You are talking about drinking 'coffee' in general, not a specific cup of 'the coffee' (which would require the specific object suffix to become kahveyi).