Teach Us, Teacher

Expressions of ability or potential (Lesson 41)

You can express ability or potential if you use the dictionary form of verbs, and attach to it KOTO GA DEKIMASU (can do / Literally, can do the thing of).

Let's make a sentence, using KOTO GA DEKIMASU. When you say "I can go," "I" is WATASHI. "To go" is IKIMASU. And its dictionary form is IKU. To this, you add KOTO GA DEKIMASU. So, you say WATASHI WA IKU KOTO GA DEKIMASU (I can go / Literally, I can do the thing of going). To turn this into a negative, you change DEKIMASU to DEKIMASEN. So, "I can't go" is WATASHI WA IKU KOTO GA DEKIMASEN.
You might remember that we also learned the potential form of verbs, expressing that "you can do something" in Lesson 35.

The potential form of IKIMASU (to go) is IKEMASU (can go). It basically means the same thing as IKU KOTO GA DEKIMASU. But IKEMASU is more casual.