しか and だけ have nearly/basically the same meaning, but are used in different grammatical situations. I think I used to understand the differences, but can’t remember at all right now.
Update: some examples from Tatoeba
Worthy of note is that in these examples, しか is consistently used with negative verb conjugation.
待つしかない。 [まつ しか ない。]: The only thing we can do is wait.
今しかない。 [いま しか ない。]: It’s now or never.
友人は少ししかいない。 [ゆうじんはすこし しか いない。]: I have few friends.
彼女は野菜しか食べない。 [かのじょはやさい しか たべない。]: She eats nothing but vegetables.
This next example uses both だけ and しか together to create meaning is “only”. I’m curious if this is proper usage.
名前だけでしか彼を知らない。 [なまえ だけ で しか かれ を しらない。]: I only know him by name.
来て下さるだけでいいのです。 [くて くださる だけでいい のです。]: I just want you to come.
彼女は、親切なだけでなく正直だ。 [かのじょは、しんせつな だけでなく しょうじき だ。]: She is not only kind but honest.
丸 and 円 have the nearly the same meaning, which is round or circular in the 2-dimensional sense. I’ve never known or understood when to use one over the other. On signs, and in basic reading opportunities that I find, I see 丸 being used to mean round or circular, and I only see 円 being used to refer to yen. I’m not sure which, but I believe one of these kanji can also be used to mean sphere, as well as 2-dimensional round things.
She (し・止) stops (止) while he (ひ・引) pulls (引) the door.