Saying what you can do with the potential form, what is visible/audible, and listing reasons with し.
The potential form expresses ability: 話す→話せる, 食べる→食べられる, する→できる.
わたしは にほんごが はなせます。
I can speak Japanese.
Group I: change the final う-row sound to the え-row + る (書く→書ける, 飲む→飲める). Group II: ます-stem + られる (食べる→食べられる). Irregular: する→できる, 来る→来られる.
The object of a potential verb is usually marked with が (not を): 日本語が 話せます.
Spontaneous perception (without effort): something is visible (見えます) or audible (聞こえます).
へやから うみが みえます。
You can see the sea from the room.
見えます/聞こえます describe what naturally comes into view or hearing, not a deliberate ability (which would be 見られます/聞けます). The thing perceived takes が.
し joins clauses to pile up reasons: “it's A, and B (too), so…”.
このみせは やすいし、おいしいです。
This shop is cheap, and the food is good.
し attaches to the plain form and lists reasons or qualities, implying there are several. It often leads to a conclusion (…から / だから).