Learn Dutch: Hij bouwt aan zijn eigen succesvolle onderneming. - He is building his own successful enterprise.

Zijn eigen emphasizes that the business is his own. The word eigen is a special adjective because it never gets an -e at the end, regardless of whether the noun is a de or het word. So it is always zijn eigen, never zijn eigene.

Why does the sentence use bouwt aan instead of just bouwt?

Bouwen aan is a fixed combination meaning to work on developing or improving something over time, usually something abstract like a business, a future, or a relationship. If you just use bouwt, it usually implies physically constructing a building or object.

Since eigen does not get an -e, why does succesvolle get one?

The noun onderneming is a de word. When an adjective comes before a de word, it normally gets an -e ending. Because eigen is an exception, the regular rule simply applies to the next adjective, turning succesvol into succesvolle. Notice that the single l becomes a double ll to keep the short o vowel sound.

Is there a difference between an onderneming and a bedrijf?