"I am slow."
First of all, I’m holding back all urges to make a pun about you being slow. :)
To analyze this, we can first separate out the bits of each:
- 저는→저+는 (I+topic marker)
- 제가→저+이+가 (I+subject marker+subject marker)
As you can see, they are really the same, differing only in the marker affixed. In many situations, there is no difference since the topic of the sentence can also be the subject. The translation will also be identical.
However, 는/은 as a topic marker can be used in a bunch of other situations:
- Comparison/contrast: I’m slow, but you are fast!
- Generalization: Men do not cry.
이/가 also has other nuances:
- Stress: I killed Mufasa!
- Answer to a who question when who is the subject: Who did it? I.
- Subject marker when there is a topic marker in the same sentence
In many cases, the nuance exists only in the right context. This cannot be taught by Duolingo when the sentences do not have any.