As you mentioned, 이젠 is a contraction of 이제는, composed of 이제 and the particle 는. 은/는 is widely used as an auxiliary particle to modify other particles, in addition to being a topic particle. It can also attach to many adverbs or replace the object particle 을/를. In such usages, it adds the contrast effect of setting it against other implied cases to sharpen the meaning. Thus 이제 is plain "now" or "from now on", but 이제는 makes it stronger, like "whatever happened before, now ...". It emphasizes the new situation more than the simple 이제. Here are some examples of such 은/는 usages where the effect is even greater: 1. 처음 봤을 때 좋은 인상: A good impression at first sight (no special nuance). 2. 처음 봤을 때는 좋은 인상: A good impression at first sight ... (but not lasting for long). 3. 입에 맞는 음식: a food delicious to my taste. 4. 입에는 맞는 음식: a food delicious to my taste ... (but probably no good health-wise). 5. 나는 수학을 잘해: I am good at math. 6. 나는 수학은 잘해: I am good at math (but not the other subjects). * for the object marker 을/를, 은/는 replaces it instead of attaching to it as with other particles. It's a special case.