Learn Finnish: Syön säännöllisesti, jotta olen energinen koko päivän. - I eat regularly so that I am energetic all day.

The word jotta means 'so that' or 'in order that' and shows the purpose of your action. In Finnish, you just follow it with a normal verb in the standard indicative form. Here, it is simply olen (I am), matching the 'I' subject from the first half of the sentence.

Why is the time expression 'koko päivän' instead of the basic form 'koko päivä'?

When you express how long an action or state lasts, the time word usually takes the accusative ending -n. This changes päivä to päivän to mean 'for the whole day'. Notice that the word koko (whole) is a special descriptive word that never changes its form to match the noun.

How do we get the adverb 'säännöllisesti' from the adjective 'säännöllinen' (regular)?

To turn an adjective ending in -nen into an adverb (similar to adding '-ly' in English), you replace the -nen ending with -sesti. This is a very common pattern, turning säännöllinen into säännöllisesti (regularly).

Could I just use 'että' (that) instead of 'jotta' in this sentence?

While you might hear että used this way in casual spoken Finnish, jotta is the correct word when you specifically want to express purpose. Using jotta makes it crystal clear that having energy is the exact reason why you eat regularly.