"If he is drinking, I am eating."
Translation:Jeśli on pije, ja jem.
Emphasis here is important, we have contrast between subjects. If we have a sentence with two subjects, those need to be said, especially second one.
Jem, on pije - Ok
Antek does something. Pije, ja jem. - ok
Ja jem ,on pije/ On pije , ja jem ok, more accent put on contrest between subjects
Thanks! (And since you are now on level 15 in Russian, I can tell you that "yezheli" also exists in Russian, but it sounds fancy, poetic, and outdated)